:sectnums:
|
:sectnums:
|
== NEORV32 Central Processing Unit (CPU)
|
== NEORV32 Central Processing Unit (CPU)
|
|
|
image::riscv_logo.png[width=350,align=center]
|
image::riscv_logo.png[width=350,align=center]
|
|
|
**Key Features**
|
**Key Features**
|
|
|
* 32-bit multi-cycle in-order `rv32` RISC-V CPU
|
* 32-bit multi-cycle in-order `rv32` RISC-V CPU
|
* Optional RISC-V extensions:
|
* Optional RISC-V extensions:
|
** `A` - atomic memory access operations
|
** `A` - atomic memory access operations
|
** `B` - bit-manipulation instructions
|
** `B` - bit-manipulation instructions
|
** `C` - 16-bit compressed instructions
|
** `C` - 16-bit compressed instructions
|
** `I` - integer base ISA (always enabled)
|
** `I` - integer base ISA (always enabled)
|
** `E` - embedded CPU version (reduced register file size)
|
** `E` - embedded CPU version (reduced register file size)
|
** `M` - integer multiplication and division hardware
|
** `M` - integer multiplication and division hardware
|
** `U` - less-privileged _user_ mode
|
** `U` - less-privileged _user_ mode
|
** `Zfinx` - single-precision floating-point unit
|
** `Zfinx` - single-precision floating-point unit
|
** `Zicsr` - control and status register access (privileged architecture)
|
** `Zicsr` - control and status register access (privileged architecture)
|
** `Zicntr` - CPU base counters
|
** `Zicntr` - CPU base counters
|
** `Zihpm` - hardware performance monitors
|
** `Zihpm` - hardware performance monitors
|
** `Zifencei` - instruction stream synchronization
|
** `Zifencei` - instruction stream synchronization
|
** `Zmmul` - integer multiplication hardware
|
** `Zmmul` - integer multiplication hardware
|
** `PMP` - physical memory protection
|
** `PMP` - physical memory protection
|
** `Debug` - debug mode
|
** `Debug` - debug mode
|
* Compatible to the RISC-V user specifications and a subset of the RISC-V privileged architecture specifications - passes the official RISC-V Architecture Tests (v2+)
|
* Compatible to the RISC-V user specifications and a subset of the RISC-V privileged architecture specifications - passes the official RISC-V Architecture Tests (v2+)
|
* Official RISC-V open-source architecture ID
|
* Official RISC-V open-source architecture ID
|
* Standard RISC-V interrupts (_external_, _timer_, _software_) plus 16 _fast_ interrupts
|
* Standard RISC-V interrupts (_external_, _timer_, _software_) plus 16 _fast_ interrupts
|
* Supports _all_ of the machine-level traps from the RISC-V specifications (including bus access exceptions and all unimplemented/illegal/malformed instructions)
|
* Supports _all_ of the machine-level traps from the RISC-V specifications (including bus access exceptions and all unimplemented/illegal/malformed instructions)
|
** This is a special aspect on _execution safety_ by <<_full_virtualization>>
|
** This is a special aspect on _execution safety_ by <<_full_virtualization>>
|
* Optional physical memory configuration (PMP), compatible to the RISC-V specifications
|
* Optional physical memory configuration (PMP), compatible to the RISC-V specifications
|
* Optional hardware performance monitors (HPM) for application benchmarking
|
* Optional hardware performance monitors (HPM) for application benchmarking
|
* Separated interfaces for instruction fetch and data access (merged into a single processor bus))
|
* Separated interfaces for instruction fetch and data access (merged into a single processor bus))
|
* little-endian byte order
|
* little-endian byte order
|
* Configurable hardware reset
|
* Configurable hardware reset
|
* No hardware support of unaligned data/instruction accesses - they will trigger an exception.
|
* No hardware support of unaligned data/instruction accesses - they will trigger an exception.
|
|
|
[NOTE]
|
[NOTE]
|
It is recommended to use the **NEORV32 Processor** as default top instance even if you only want to use the actual
|
It is recommended to use the **NEORV32 Processor** as default top instance even if you only want to use the actual
|
CPU. Simply disable all the processor-internal modules via the generics and you will get a "CPU
|
CPU. Simply disable all the processor-internal modules via the generics and you will get a "CPU
|
wrapper" that provides a minimal CPU environment and an external bus interface (like AXI4). This
|
wrapper" that provides a minimal CPU environment and an external bus interface (like AXI4). This
|
setup also allows to further use the default bootloader and software framework. From this base you
|
setup also allows to further use the default bootloader and software framework. From this base you
|
can start building your own SoC. Of course you can also use the CPU in it’s true stand-alone mode.
|
can start building your own SoC. Of course you can also use the CPU in it’s true stand-alone mode.
|
|
|
[NOTE]
|
[NOTE]
|
This documentation assumes the reader is familiar with the official RISC-V "User" and "Privileged Architecture" specifications.
|
This documentation assumes the reader is familiar with the official RISC-V "User" and "Privileged Architecture" specifications.
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== Architecture
|
=== Architecture
|
|
|
The NEORV32 CPU was designed from scratch based only on the official ISA / privileged architecture
|
The NEORV32 CPU was designed from scratch based only on the official ISA / privileged architecture
|
specifications. The following figure shows the simplified architecture of the CPU.
|
specifications. The following figure shows the simplified architecture of the CPU.
|
|
|
image::neorv32_cpu.png[align=center]
|
image::neorv32_cpu.png[align=center]
|
|
|
The CPU implements a _multi-cycle_ architecture. Hence, each instruction is executed as a series of consecutive
|
The CPU implements a _multi-cycle_ architecture. Hence, each instruction is executed as a series of consecutive
|
micro-operations. In order to increase performance, the CPU's **front-end** (instruction fetch) and **back-end**
|
micro-operations. In order to increase performance, the CPU's **front-end** (instruction fetch) and **back-end**
|
(instruction execution) are de-couples via a FIFO (the "instruction prefetch buffer"). Therefore, the
|
(instruction execution) are de-couples via a FIFO (the "instruction prefetch buffer"). Therefore, the
|
front-end can already fetch new instructions while the back-end is still processing previously-fetched instructions.
|
front-end can already fetch new instructions while the back-end is still processing previously-fetched instructions.
|
|
|
The front-end is responsible for fetching 32-bit chunks of instruction words (one aligned 32-bit instruction,
|
The front-end is responsible for fetching 32-bit chunks of instruction words (one aligned 32-bit instruction,
|
two 16-bit instructions or a mixture if 32-bit instructions are not aligned to 32-bit boundaries). The instruction
|
two 16-bit instructions or a mixture if 32-bit instructions are not aligned to 32-bit boundaries). The instruction
|
data is stored to a FIFO queue - the instruction prefetch buffer.
|
data is stored to a FIFO queue - the instruction prefetch buffer.
|
|
|
The back-end is responsible for the actual execution of the instruction. It includes an "issue engine",
|
The back-end is responsible for the actual execution of the instruction. It includes an "issue engine",
|
which takes data from the instruction prefetch buffer and assembles 32-bit instruction words (plain 32-bit
|
which takes data from the instruction prefetch buffer and assembles 32-bit instruction words (plain 32-bit
|
instruction or decompressed 16-bit instructions) for execution.
|
instruction or decompressed 16-bit instructions) for execution.
|
|
|
Front-end and back-end operate in parallel and with overlapping operations. Hence, the optimal CPI
|
Front-end and back-end operate in parallel and with overlapping operations. Hence, the optimal CPI
|
(cycles per instructions) is 2, but it can be significantly higher: for instance when executing loads/stores
|
(cycles per instructions) is 2, but it can be significantly higher: for instance when executing loads/stores
|
(accessing memory-mapped devices with high latency), executing multi-cycle ALU operations (like divisions) or
|
(accessing memory-mapped devices with high latency), executing multi-cycle ALU operations (like divisions) or
|
when the CPU front-end has to reload the prefetch buffer due to a taken branch.
|
when the CPU front-end has to reload the prefetch buffer due to a taken branch.
|
|
|
Basically, the NEORV32 CPU is somewhere between a classical pipelined architecture, where each stage
|
Basically, the NEORV32 CPU is somewhere between a classical pipelined architecture, where each stage
|
requires exactly one processing cycle (if not stalled) and a classical multi-cycle architecture, which executes
|
requires exactly one processing cycle (if not stalled) and a classical multi-cycle architecture, which executes
|
every single instruction (_including_ fetch) in a series of consecutive micro-operations. The combination of
|
every single instruction (_including_ fetch) in a series of consecutive micro-operations. The combination of
|
these two classical design paradigms allows an increased instruction execution in contrast to a pure multi-cycle
|
these two classical design paradigms allows an increased instruction execution in contrast to a pure multi-cycle
|
approach (due to overlapping operation of fetch and execute) at a reduced hardware footprint (due to the
|
approach (due to overlapping operation of fetch and execute) at a reduced hardware footprint (due to the
|
multi-cycle concept).
|
multi-cycle concept).
|
|
|
As a Von-Neumann machine, the CPU provides independent interfaces for instruction fetch and data access.
|
As a Von-Neumann machine, the CPU provides independent interfaces for instruction fetch and data access.
|
These two bus interfaces are merged into a single processor-internal bus via a prioritizing bus switch (data accesses
|
These two bus interfaces are merged into a single processor-internal bus via a prioritizing bus switch (data accesses
|
have higher priority). Hence, ALL memory locations including peripheral devices are mapped to a single unified 32-bit
|
have higher priority). Hence, ALL memory locations including peripheral devices are mapped to a single unified 32-bit
|
address space.
|
address space.
|
|
|
|
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== Full Virtualization
|
=== Full Virtualization
|
|
|
Just like the RISC-V ISA the NEORV32 aims to provide _maximum virtualization_ capabilities on CPU _and_ SoC level to
|
Just like the RISC-V ISA the NEORV32 aims to provide _maximum virtualization_ capabilities on CPU _and_ SoC level to
|
allow a high standard of **execution safety**. The CPU supports **all** traps specified by the official RISC-V specifications.
|
allow a high standard of **execution safety**. The CPU supports **all** traps specified by the official RISC-V specifications.
|
footnote:[If the `Zicsr` CPU extension is enabled (implementing the full set of the privileged architecture).]
|
footnote:[If the `Zicsr` CPU extension is enabled (implementing the full set of the privileged architecture).]
|
Thus, the CPU provides defined hardware fall-backs via traps for any expected and unexpected situation (e.g. executing an
|
Thus, the CPU provides defined hardware fall-backs via traps for any expected and unexpected situation (e.g. executing an
|
malformed instruction word or accessing a not-allocated memory address). For any kind of trap the core is always in a
|
malformed instruction word or accessing a not-allocated memory address). For any kind of trap the core is always in a
|
defined and fully synchronized state throughout the whole architecture (i.e. there are no out-of-order operations that
|
defined and fully synchronized state throughout the whole architecture (i.e. there are no out-of-order operations that
|
might have to reverted). This allows predictable execution behavior at any time improving overall _execution safety_.
|
might have to reverted). This allows predictable execution behavior at any time improving overall _execution safety_.
|
|
|
**Execution Safety - NEORV32 Virtualization Features**
|
**Execution Safety - NEORV32 Virtualization Features**
|
|
|
* Due to the acknowledged memory accesses the CPU is _always_ sync with the memory system
|
* Due to the acknowledged memory accesses the CPU is _always_ sync with the memory system
|
(i.e. there is no speculative execution / no out-of-order states).
|
(i.e. there is no speculative execution / no out-of-order states).
|
* The CPU supports _all_ RISC-V compatible bus exceptions including access exceptions, which are triggered if an
|
* The CPU supports _all_ RISC-V compatible bus exceptions including access exceptions, which are triggered if an
|
accessed address does not respond or encounters an internal error during access.
|
accessed address does not respond or encounters an internal error during access.
|
* Accessed memory addresses (plain memory, but also memory-mapped devices) need to respond within a fixed time
|
* Accessed memory addresses (plain memory, but also memory-mapped devices) need to respond within a fixed time
|
window. Otherwise a bus access exception is raised.
|
window. Otherwise a bus access exception is raised.
|
* The RISC-V specs. state that executing an malformed instruction results in unpredictable behavior. As an additional
|
* The RISC-V specs. state that executing an malformed instruction results in unpredictable behavior. As an additional
|
execution safety feature the NEORV32 CPU ensures that _all_ unimplemented/malformed/illegal instructions do raise an
|
execution safety feature the NEORV32 CPU ensures that _all_ unimplemented/malformed/illegal instructions do raise an
|
illegal instruction exceptions and do not commit any state-changing operation (like writing registers or triggering
|
illegal instruction exceptions and do not commit any state-changing operation (like writing registers or triggering
|
memory operations).
|
memory operations).
|
* To be continued...
|
* To be continued...
|
|
|
|
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== RISC-V Compatibility
|
=== RISC-V Compatibility
|
|
|
The NEORV32 CPU passes the rv32_m/I, rv32_m/M, rv32_m/C, rv32_m/privilege, and
|
The NEORV32 CPU passes the rv32_m/I, rv32_m/M, rv32_m/C, rv32_m/privilege, and
|
rv32_m/Zifencei tests of the official RISC-V Architecture Tests (GitHub). The port files for the
|
rv32_m/Zifencei tests of the official RISC-V Architecture Tests (GitHub). The port files for the
|
NEORV32 processor are located in the repository's `sw/isa-test` folder.
|
NEORV32 processor are located in the repository's `sw/isa-test` folder.
|
|
|
[NOTE]
|
[NOTE]
|
See section https://stnolting.github.io/neorv32/ug/#_risc_v_architecture_test_framework[User Guide: RISC-V Architecture Test Framework]
|
See section https://stnolting.github.io/neorv32/ug/#_risc_v_architecture_test_framework[User Guide: RISC-V Architecture Test Framework]
|
for information how to run the tests on the NEORV32.
|
for information how to run the tests on the NEORV32.
|
|
|
.**RISC-V `rv32_m/C` Tests**
|
.**RISC-V `rv32_m/C` Tests**
|
...................................
|
...................................
|
Check cadd-01 ... OK
|
Check cadd-01 ... OK
|
Check caddi-01 ... OK
|
Check caddi-01 ... OK
|
Check caddi16sp-01 ... OK
|
Check caddi16sp-01 ... OK
|
Check caddi4spn-01 ... OK
|
Check caddi4spn-01 ... OK
|
Check cand-01 ... OK
|
Check cand-01 ... OK
|
Check candi-01 ... OK
|
Check candi-01 ... OK
|
Check cbeqz-01 ... OK
|
Check cbeqz-01 ... OK
|
Check cbnez-01 ... OK
|
Check cbnez-01 ... OK
|
Check cebreak-01 ... OK
|
Check cebreak-01 ... OK
|
Check cj-01 ... OK
|
Check cj-01 ... OK
|
Check cjal-01 ... OK
|
Check cjal-01 ... OK
|
Check cjalr-01 ... OK
|
Check cjalr-01 ... OK
|
Check cjr-01 ... OK
|
Check cjr-01 ... OK
|
Check cli-01 ... OK
|
Check cli-01 ... OK
|
Check clui-01 ... OK
|
Check clui-01 ... OK
|
Check clw-01 ... OK
|
Check clw-01 ... OK
|
Check clwsp-01 ... OK
|
Check clwsp-01 ... OK
|
Check cmv-01 ... OK
|
Check cmv-01 ... OK
|
Check cnop-01 ... OK
|
Check cnop-01 ... OK
|
Check cor-01 ... OK
|
Check cor-01 ... OK
|
Check cslli-01 ... OK
|
Check cslli-01 ... OK
|
Check csrai-01 ... OK
|
Check csrai-01 ... OK
|
Check csrli-01 ... OK
|
Check csrli-01 ... OK
|
Check csub-01 ... OK
|
Check csub-01 ... OK
|
Check csw-01 ... OK
|
Check csw-01 ... OK
|
Check cswsp-01 ... OK
|
Check cswsp-01 ... OK
|
Check cxor-01 ... OK
|
Check cxor-01 ... OK
|
--------------------------------
|
--------------------------------
|
OK: 27/27 RISCV_TARGET=neorv32 RISCV_DEVICE=C XLEN=32
|
OK: 27/27 RISCV_TARGET=neorv32 RISCV_DEVICE=C XLEN=32
|
...................................
|
...................................
|
|
|
.**RISC-V `rv32_m/I` Tests**
|
.**RISC-V `rv32_m/I` Tests**
|
...................................
|
...................................
|
Check add-01 ... OK
|
Check add-01 ... OK
|
Check addi-01 ... OK
|
Check addi-01 ... OK
|
Check and-01 ... OK
|
Check and-01 ... OK
|
Check andi-01 ... OK
|
Check andi-01 ... OK
|
Check auipc-01 ... OK
|
Check auipc-01 ... OK
|
Check beq-01 ... OK
|
Check beq-01 ... OK
|
Check bge-01 ... OK
|
Check bge-01 ... OK
|
Check bgeu-01 ... OK
|
Check bgeu-01 ... OK
|
Check blt-01 ... OK
|
Check blt-01 ... OK
|
Check bltu-01 ... OK
|
Check bltu-01 ... OK
|
Check bne-01 ... OK
|
Check bne-01 ... OK
|
Check fence-01 ... OK
|
Check fence-01 ... OK
|
Check jal-01 ... OK
|
Check jal-01 ... OK
|
Check jalr-01 ... OK
|
Check jalr-01 ... OK
|
Check lb-align-01 ... OK
|
Check lb-align-01 ... OK
|
Check lbu-align-01 ... OK
|
Check lbu-align-01 ... OK
|
Check lh-align-01 ... OK
|
Check lh-align-01 ... OK
|
Check lhu-align-01 ... OK
|
Check lhu-align-01 ... OK
|
Check lui-01 ... OK
|
Check lui-01 ... OK
|
Check lw-align-01 ... OK
|
Check lw-align-01 ... OK
|
Check or-01 ... OK
|
Check or-01 ... OK
|
Check ori-01 ... OK
|
Check ori-01 ... OK
|
Check sb-align-01 ... OK
|
Check sb-align-01 ... OK
|
Check sh-align-01 ... OK
|
Check sh-align-01 ... OK
|
Check sll-01 ... OK
|
Check sll-01 ... OK
|
Check slli-01 ... OK
|
Check slli-01 ... OK
|
Check slt-01 ... OK
|
Check slt-01 ... OK
|
Check slti-01 ... OK
|
Check slti-01 ... OK
|
Check sltiu-01 ... OK
|
Check sltiu-01 ... OK
|
Check sltu-01 ... OK
|
Check sltu-01 ... OK
|
Check sra-01 ... OK
|
Check sra-01 ... OK
|
Check srai-01 ... OK
|
Check srai-01 ... OK
|
Check srl-01 ... OK
|
Check srl-01 ... OK
|
Check srli-01 ... OK
|
Check srli-01 ... OK
|
Check sub-01 ... OK
|
Check sub-01 ... OK
|
Check sw-align-01 ... OK
|
Check sw-align-01 ... OK
|
Check xor-01 ... OK
|
Check xor-01 ... OK
|
Check xori-01 ... OK
|
Check xori-01 ... OK
|
--------------------------------
|
--------------------------------
|
OK: 38/38 RISCV_TARGET=neorv32 RISCV_DEVICE=I XLEN=32
|
OK: 38/38 RISCV_TARGET=neorv32 RISCV_DEVICE=I XLEN=32
|
...................................
|
...................................
|
|
|
.**RISC-V `rv32_m/M` Tests**
|
.**RISC-V `rv32_m/M` Tests**
|
...................................
|
...................................
|
Check div-01 ... OK
|
Check div-01 ... OK
|
Check divu-01 ... OK
|
Check divu-01 ... OK
|
Check mul-01 ... OK
|
Check mul-01 ... OK
|
Check mulh-01 ... OK
|
Check mulh-01 ... OK
|
Check mulhsu-01 ... OK
|
Check mulhsu-01 ... OK
|
Check mulhu-01 ... OK
|
Check mulhu-01 ... OK
|
Check rem-01 ... OK
|
Check rem-01 ... OK
|
Check remu-01 ... OK
|
Check remu-01 ... OK
|
--------------------------------
|
--------------------------------
|
OK: 8/8 RISCV_TARGET=neorv32 RISCV_DEVICE=M XLEN=32
|
OK: 8/8 RISCV_TARGET=neorv32 RISCV_DEVICE=M XLEN=32
|
...................................
|
...................................
|
|
|
.**RISC-V `rv32_m/privilege` Tests**
|
.**RISC-V `rv32_m/privilege` Tests**
|
...................................
|
...................................
|
Check ebreak ... OK
|
Check ebreak ... OK
|
Check ecall ... OK
|
Check ecall ... OK
|
Check misalign-beq-01 ... OK
|
Check misalign-beq-01 ... OK
|
Check misalign-bge-01 ... OK
|
Check misalign-bge-01 ... OK
|
Check misalign-bgeu-01 ... OK
|
Check misalign-bgeu-01 ... OK
|
Check misalign-blt-01 ... OK
|
Check misalign-blt-01 ... OK
|
Check misalign-bltu-01 ... OK
|
Check misalign-bltu-01 ... OK
|
Check misalign-bne-01 ... OK
|
Check misalign-bne-01 ... OK
|
Check misalign-jal-01 ... OK
|
Check misalign-jal-01 ... OK
|
Check misalign-lh-01 ... OK
|
Check misalign-lh-01 ... OK
|
Check misalign-lhu-01 ... OK
|
Check misalign-lhu-01 ... OK
|
Check misalign-lw-01 ... OK
|
Check misalign-lw-01 ... OK
|
Check misalign-sh-01 ... OK
|
Check misalign-sh-01 ... OK
|
Check misalign-sw-01 ... OK
|
Check misalign-sw-01 ... OK
|
Check misalign1-jalr-01 ... OK
|
Check misalign1-jalr-01 ... OK
|
Check misalign2-jalr-01 ... OK
|
Check misalign2-jalr-01 ... OK
|
--------------------------------
|
--------------------------------
|
OK: 16/16 RISCV_TARGET=neorv32 RISCV_DEVICE=privilege XLEN=32
|
OK: 16/16 RISCV_TARGET=neorv32 RISCV_DEVICE=privilege XLEN=32
|
...................................
|
...................................
|
|
|
.**RISC-V `rv32_m/Zifencei` Tests**
|
.**RISC-V `rv32_m/Zifencei` Tests**
|
...................................
|
...................................
|
Check Fencei ... OK
|
Check Fencei ... OK
|
--------------------------------
|
--------------------------------
|
OK: 1/1 RISCV_TARGET=neorv32 RISCV_DEVICE=Zifencei XLEN=32
|
OK: 1/1 RISCV_TARGET=neorv32 RISCV_DEVICE=Zifencei XLEN=32
|
...................................
|
...................................
|
|
|
|
|
<<<
|
<<<
|
:sectnums:
|
:sectnums:
|
==== RISC-V Incompatibility Issues and Limitations
|
==== RISC-V Incompatibility Issues and Limitations
|
|
|
This list shows the currently identified issues regarding full RISC-V-compatibility. More specific information
|
This list shows the currently identified issues regarding full RISC-V-compatibility. More specific information
|
can be found in section <<_instruction_sets_and_extensions>>.
|
can be found in section <<_instruction_sets_and_extensions>>.
|
|
|
.Hardwired R/W CSRs
|
.Read-Only "Read-Write" CSRs
|
[IMPORTANT]
|
[IMPORTANT]
|
The `misa`, `mip` and `mtval` CSRs in the NEORV32 are _read-only_.
|
The `misa` and `mtval` CSRs in the NEORV32 are _read-only_.
|
Any write access to it (in machine mode) to them are ignored and will _not_ cause any exceptions or side-effects.
|
Any machine-mode write access to them is ignored and will _not_ cause any exceptions or side-effects to maintain
|
Pending interrupt can only be cleared by acknowledging the interrupt-causing device. However, pending interrupts
|
RISC-V compatibility.
|
can still be ignored by clearing the according `mie` register bits.
|
|
|
|
.Physical memory protection
|
.Physical Memory Protection
|
[IMPORTANT]
|
[IMPORTANT]
|
The physical memory protection (see section <<_machine_physical_memory_protection>>)
|
The physical memory protection (see section <<_machine_physical_memory_protection>>)
|
only supports the modes _OFF_ and _NAPOT_ yet and a minimal granularity of 8 bytes per region.
|
only supports the modes _OFF_ and _NAPOT_ yet and a minimal granularity of 8 bytes per region.
|
|
|
.Atomic memory operations
|
.Atomic Memory Operations
|
[IMPORTANT]
|
[IMPORTANT]
|
The `A` CPU extension only implements the `lr.w` and `sc.w` instructions yet.
|
The `A` CPU extension only implements the `lr.w` and `sc.w` instructions yet.
|
However, these instructions are sufficient to emulate all further atomic memory operations.
|
However, these instructions are sufficient to emulate all further atomic memory operations.
|
|
|
.Bit-manipulation operations
|
.Bit-Manipulation ISA Extension
|
[IMPORTANT]
|
[IMPORTANT]
|
The NEORV32 `B` extension only implements the _basic bit-manipulation instructions_ (`Zbb`) subset
|
The NEORV32 `B` extension only implements the _basic bit-manipulation instructions_ (`Zbb`) subset
|
and the _address generation instructions_ (`Zba`) subset yet.
|
and the _address generation instructions_ (`Zba`) subset yet.
|
|
|
.Instruction Misalignment
|
|
[NOTE]
|
|
This is not a real RISC-V incompatibility, but something that might not be clear when studying the RISC-V privileged
|
|
architecture specifications: for 32-bit only instructions (no `C` extension) the misaligned instruction exception
|
|
is raised if bit 1 of the access address is set (i.e. not on 32-bit boundary). If the `C` extension is implemented
|
|
there will be no misaligned instruction exceptions _at all_.
|
|
In both cases bit 0 of the program counter and all related registers is hardwired to zero.
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== CPU Top Entity - Signals
|
=== CPU Top Entity - Signals
|
|
|
The following table shows all interface signals of the CPU top entity `rtl/core/neorv32_cpu.vhd`. The
|
The following table shows all interface signals of the CPU top entity `rtl/core/neorv32_cpu.vhd`. The
|
type of all signals is _std_ulogic_ or _std_ulogic_vector_, respectively. The "Dir." column shows the signal
|
type of all signals is _std_ulogic_ or _std_ulogic_vector_, respectively. The "Dir." column shows the signal
|
direction seen from the CPU.
|
direction seen from the CPU.
|
|
|
.NEORV32 CPU top entity signals
|
.NEORV32 CPU top entity signals
|
[cols="<2,^1,^1,<6"]
|
[cols="<2,^1,^1,<6"]
|
[options="header", grid="rows"]
|
[options="header", grid="rows"]
|
|=======================
|
|=======================
|
| Signal | Width | Dir. | Function
|
| Signal | Width | Dir. | Function
|
4+^| **Global Signals**
|
4+^| **Global Signals**
|
| `clk_i` | 1 | in | global clock line, all registers triggering on rising edge
|
| `clk_i` | 1 | in | global clock line, all registers triggering on rising edge
|
| `rstn_i` | 1 | in | global reset, low-active
|
| `rstn_i` | 1 | in | global reset, low-active
|
| `sleep_o` | 1 | out | CPU is in sleep mode when set
|
| `sleep_o` | 1 | out | CPU is in sleep mode when set
|
|
| `debug_o` | 1 | out | CPU is in debug mode when set
|
4+^| **Instruction Bus Interface (<<_bus_interface>>)**
|
4+^| **Instruction Bus Interface (<<_bus_interface>>)**
|
| `i_bus_addr_o` | 32 | out | destination address
|
| `i_bus_addr_o` | 32 | out | destination address
|
| `i_bus_rdata_i` | 32 | in | read data
|
| `i_bus_rdata_i` | 32 | in | read data
|
| `i_bus_wdata_o` | 32 | out | write data (always zero)
|
| `i_bus_wdata_o` | 32 | out | write data (always zero)
|
| `i_bus_ben_o` | 4 | out | byte enable
|
| `i_bus_ben_o` | 4 | out | byte enable
|
| `i_bus_we_o` | 1 | out | write transaction (always zero)
|
| `i_bus_we_o` | 1 | out | write transaction (always zero)
|
| `i_bus_re_o` | 1 | out | read transaction
|
| `i_bus_re_o` | 1 | out | read transaction
|
| `i_bus_lock_o` | 1 | out | exclusive access request (always zero)
|
| `i_bus_lock_o` | 1 | out | exclusive access request (always zero)
|
| `i_bus_ack_i` | 1 | in | bus transfer acknowledge from accessed peripheral
|
| `i_bus_ack_i` | 1 | in | bus transfer acknowledge from accessed peripheral
|
| `i_bus_err_i` | 1 | in | bus transfer terminate from accessed peripheral
|
| `i_bus_err_i` | 1 | in | bus transfer terminate from accessed peripheral
|
| `i_bus_fence_o` | 1 | out | indicates an executed _fence.i_ instruction
|
| `i_bus_fence_o` | 1 | out | indicates an executed _fence.i_ instruction
|
| `i_bus_priv_o` | 2 | out | current CPU privilege level
|
| `i_bus_priv_o` | 2 | out | current CPU privilege level
|
4+^| **Data Bus Interface (<<_bus_interface>>)**
|
4+^| **Data Bus Interface (<<_bus_interface>>)**
|
| `d_bus_addr_o` | 32 | out | destination address
|
| `d_bus_addr_o` | 32 | out | destination address
|
| `d_bus_rdata_i` | 32 | in | read data
|
| `d_bus_rdata_i` | 32 | in | read data
|
| `d_bus_wdata_o` | 32 | out | write data
|
| `d_bus_wdata_o` | 32 | out | write data
|
| `d_bus_ben_o` | 4 | out | byte enable
|
| `d_bus_ben_o` | 4 | out | byte enable
|
| `d_bus_we_o` | 1 | out | write transaction
|
| `d_bus_we_o` | 1 | out | write transaction
|
| `d_bus_re_o` | 1 | out | read transaction
|
| `d_bus_re_o` | 1 | out | read transaction
|
| `d_bus_lock_o` | 1 | out | exclusive access request
|
| `d_bus_lock_o` | 1 | out | exclusive access request
|
| `d_bus_ack_i` | 1 | in | bus transfer acknowledge from accessed peripheral
|
| `d_bus_ack_i` | 1 | in | bus transfer acknowledge from accessed peripheral
|
| `d_bus_err_i` | 1 | in | bus transfer terminate from accessed peripheral
|
| `d_bus_err_i` | 1 | in | bus transfer terminate from accessed peripheral
|
| `d_bus_fence_o` | 1 | out | indicates an executed _fence_ instruction
|
| `d_bus_fence_o` | 1 | out | indicates an executed _fence_ instruction
|
| `d_bus_priv_o` | 2 | out | current CPU privilege level
|
| `d_bus_priv_o` | 2 | out | current CPU privilege level
|
4+^| **System Time (see <<_timeh>> CSR)**
|
4+^| **System Time (see <<_timeh>> CSR)**
|
| `time_i` | 64 | in | system time input (from MTIME)
|
| `time_i` | 64 | in | system time input (from MTIME)
|
4+^| **Interrupts, RISC-V-compatible (<<_traps_exceptions_and_interrupts>>)**
|
4+^| **Interrupts, RISC-V-compatible (<<_traps_exceptions_and_interrupts>>)**
|
| `msw_irq_i` | 1 | in | RISC-V machine software interrupt
|
| `msw_irq_i` | 1 | in | RISC-V machine software interrupt
|
| `mext_irq_i` | 1 | in | RISC-V machine external interrupt
|
| `mext_irq_i` | 1 | in | RISC-V machine external interrupt
|
| `mtime_irq_i` | 1 | in | RISC-V machine timer interrupt
|
| `mtime_irq_i` | 1 | in | RISC-V machine timer interrupt
|
4+^| **Fast Interrupts, NEORV32-specific (<<_traps_exceptions_and_interrupts>>)**
|
4+^| **Fast Interrupts, NEORV32-specific (<<_traps_exceptions_and_interrupts>>)**
|
| `firq_i` | 16 | in | fast interrupt request signals
|
| `firq_i` | 16 | in | fast interrupt request signals
|
4+^| **Enter Debug Mode Request (<<_on_chip_debugger_ocd>>)**
|
4+^| **Enter Debug Mode Request (<<_on_chip_debugger_ocd>>)**
|
| `db_halt_req_i` | 1 | in | request CPU to halt and enter debug mode
|
| `db_halt_req_i` | 1 | in | request CPU to halt and enter debug mode
|
|=======================
|
|=======================
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== CPU Top Entity - Generics
|
=== CPU Top Entity - Generics
|
|
|
Most of the CPU configuration generics are a subset of the actual Processor configuration generics (see section <<_processor_top_entity_generics>>).
|
Most of the CPU configuration generics are a subset of the actual Processor configuration generics (see section <<_processor_top_entity_generics>>).
|
and are not listed here. However, the CPU provides some _specific_ generics that are used to configure the CPU for the
|
and are not listed here. However, the CPU provides some _specific_ generics that are used to configure the CPU for the
|
NEORV32 processor setup. These generics are assigned by the processor setup only and are not available for user defined configuration.
|
NEORV32 processor setup. These generics are assigned by the processor setup only and are not available for user defined configuration.
|
The _specific_ generics are listed below.
|
The _specific_ generics are listed below.
|
|
|
[cols="4,4,2"]
|
[cols="4,4,2"]
|
[frame="all",grid="none"]
|
[frame="all",grid="none"]
|
|======
|
|======
|
| **CPU_BOOT_ADDR** | _std_ulogic_vector(31 downto 0)_ | 0x00000000
|
| **CPU_BOOT_ADDR** | _std_ulogic_vector(31 downto 0)_ | 0x00000000
|
3+| This address defines the reset address at which the CPU starts fetching instructions after reset. In terms of the NEORV32 processor, this
|
3+| This address defines the reset address at which the CPU starts fetching instructions after reset. In terms of the NEORV32 processor, this
|
generic is configured with the base address of the bootloader ROM (default) or with the base address of the processor-internal instruction
|
generic is configured with the base address of the bootloader ROM (default) or with the base address of the processor-internal instruction
|
memory (IMEM) if the bootloader is disabled (_INT_BOOTLOADER_EN_ = _false_). See section <<_address_space>> for more information.
|
memory (IMEM) if the bootloader is disabled (_INT_BOOTLOADER_EN_ = _false_). See section <<_address_space>> for more information.
|
|======
|
|======
|
|
|
[cols="4,4,2"]
|
[cols="4,4,2"]
|
[frame="all",grid="none"]
|
[frame="all",grid="none"]
|
|======
|
|======
|
| **CPU_DEBUG_ADDR** | _std_ulogic_vector(31 downto 0)_ | 0x00000000
|
| **CPU_DEBUG_ADDR** | _std_ulogic_vector(31 downto 0)_ | 0x00000000
|
3+| This address defines the entry address for the "execution based" on-chip debugger. By default, this generic is configured with the base address
|
3+| This address defines the entry address for the "execution based" on-chip debugger. By default, this generic is configured with the base address
|
of the debugger memory. See section <<_on_chip_debugger_ocd>> for more information.
|
of the debugger memory. See section <<_on_chip_debugger_ocd>> for more information.
|
|======
|
|======
|
|
|
[cols="4,4,2"]
|
[cols="4,4,2"]
|
[frame="all",grid="none"]
|
[frame="all",grid="none"]
|
|======
|
|======
|
| **CPU_EXTENSION_RISCV_DEBUG** | _boolean_ | false
|
| **CPU_EXTENSION_RISCV_DEBUG** | _boolean_ | false
|
3+| Implement RISC-V-compatible "debug" CPU operation mode. See section <<_cpu_debug_mode>> for more information.
|
3+| Implement RISC-V-compatible "debug" CPU operation mode. See section <<_cpu_debug_mode>> for more information.
|
|======
|
|======
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== Instruction Sets and Extensions
|
=== Instruction Sets and Extensions
|
|
|
The basic NEORV32 is a RISC-V `rv32i` architecture that provides several _optional_ RISC-V CPU and ISA
|
The basic NEORV32 is a RISC-V `rv32i` architecture that provides several _optional_ RISC-V CPU and ISA
|
(instruction set architecture) extensions. For more information regarding the RISC-V ISA extensions please
|
(instruction set architecture) extensions. For more information regarding the RISC-V ISA extensions please
|
see the the _RISC-V Instruction Set Manual - Volume I: Unprivileged ISA_ and _The RISC-V Instruction Set Manual
|
see the the _RISC-V Instruction Set Manual - Volume I: Unprivileged ISA_ and _The RISC-V Instruction Set Manual
|
Volume II: Privileged Architecture_, which are available in the projects `docs/references` folder.
|
Volume II: Privileged Architecture_, which are available in the projects `docs/references` folder.
|
|
|
[TIP]
|
[TIP]
|
The CPU can discover available ISA extensions via the <<_misa>> CSR and the
|
The CPU can discover available ISA extensions via the <<_misa>> CSR and the
|
`CPU` <<_system_configuration_information_memory_sysinfo, SYSINFO>> register
|
`CPU` <<_system_configuration_information_memory_sysinfo, SYSINFO>> register
|
or by executing an instruction and checking for an _illegal instruction exception_.
|
or by executing an instruction and checking for an _illegal instruction exception_.
|
|
|
[NOTE]
|
[NOTE]
|
Executing an instruction from an extension that is not supported yet or that is currently not enabled
|
Executing an instruction from an extension that is not supported yet or that is currently not enabled
|
(via the according top entity generic) will raise an _illegal instruction_ exception.
|
(via the according top entity generic) will raise an _illegal instruction_ exception.
|
|
|
|
|
==== **`A`** - Atomic Memory Access
|
==== **`A`** - Atomic Memory Access
|
|
|
Atomic memory access instructions allow more sophisticated memory operations like implementing semaphores and mutexes.
|
Atomic memory access instructions allow more sophisticated memory operations like implementing semaphores and mutexes.
|
The RICS-C specs. defines a specific _atomic_ extension that provides instructions for atomic memory accesses. The `A`
|
The RICS-C specs. defines a specific _atomic_ extension that provides instructions for atomic memory accesses. The `A`
|
ISA extension is enabled if the `CPU_EXTENSION_RISCV_A` configuration generic is _true_.
|
ISA extension is enabled if the `CPU_EXTENSION_RISCV_A` configuration generic is _true_.
|
In this case the following additional instructions are available:
|
In this case the following additional instructions are available:
|
|
|
* `lr.w`: load-reservate
|
* `lr.w`: load-reservate
|
* `sc.w`: store-conditional
|
* `sc.w`: store-conditional
|
|
|
[NOTE]
|
[NOTE]
|
Even though only `lr.w` and `sc.w` instructions are implemented yet, all further atomic operations
|
Even though only `lr.w` and `sc.w` instructions are implemented yet, all further atomic operations
|
(load-modify-write instruction) can be emulated using these two instruction. Furthermore, the
|
(load-modify-write instruction) can be emulated using these two instruction. Furthermore, the
|
instruction's ordering flags (`aq` and `lr`) are ignored by the CPU hardware. Using any other (not yet
|
instruction's ordering flags (`aq` and `lr`) are ignored by the CPU hardware. Using any other (not yet
|
implemented) AMO (atomic memory operation) will raise an illegal instruction exception.
|
implemented) AMO (atomic memory operation) will raise an illegal instruction exception.
|
|
|
The *load-reservate* instruction behaves as a "normal" load-word instruction (`lw`) but will also set a CPU-internal
|
The *load-reservate* instruction behaves as a "normal" load-word instruction (`lw`) but will also set a CPU-internal
|
_data memory access lock_. Executing a *store-conditional* behaves as "normal" store-word instruction (`sw`) that will
|
_data memory access lock_. Executing a *store-conditional* behaves as "normal" store-word instruction (`sw`) that will
|
only conduct an actual memory write operations if the lock is still intact. Additionally, the store-conditional instruction
|
only conduct an actual memory write operations if the lock is still intact. Additionally, the store-conditional instruction
|
will also return the lock state (returns zero if the lock is still intact or non-zero if the lock has been broken).
|
will also return the lock state (returns zero if the lock is still intact or non-zero if the lock has been broken).
|
After the execution of the `sc` instruction, the lock is automatically removed.
|
After the execution of the `sc` instruction, the lock is automatically removed.
|
|
|
The lock is broken if at least one of the following conditions occur:
|
The lock is broken if at least one of the following conditions occur:
|
. executing any data memory access instruction other than `lr.w`
|
. executing any data memory access instruction other than `lr.w`
|
. raising _any_ t (for example an interrupt or a memory access exception)
|
. raising _any_ t (for example an interrupt or a memory access exception)
|
|
|
[NOTE]
|
[NOTE]
|
The atomic instructions have special requirements for memory system / bus interconnect. More
|
The atomic instructions have special requirements for memory system / bus interconnect. More
|
information can be found in sections <<_bus_interface>> and <<_processor_external_memory_interface_wishbone_axi4_lite>>, respectively.
|
information can be found in sections <<_bus_interface>> and <<_processor_external_memory_interface_wishbone_axi4_lite>>, respectively.
|
|
|
|
|
==== **`B`** - Bit-Manipulation Operations
|
==== **`B`** - Bit-Manipulation Operations
|
|
|
The `B` ISA extension adds instructions for bit-manipulation operations. This extension is enabled if the
|
The `B` ISA extension adds instructions for bit-manipulation operations. This extension is enabled if the
|
`CPU_EXTENSION_RISCV_B` configuration generic is _true_.
|
`CPU_EXTENSION_RISCV_B` configuration generic is _true_.
|
The official RISC-V specifications can be found here: https://github.com/riscv/riscv-bitmanip
|
The official RISC-V specifications can be found here: https://github.com/riscv/riscv-bitmanip
|
|
|
[IMPORTANT]
|
[IMPORTANT]
|
The NEORV32 `B` extension only implements the _basic bit-manipulation instructions_ (`Zbb`) subset
|
The NEORV32 `B` extension only implements the _basic bit-manipulation instructions_ (`Zbb`) subset
|
and the _address generation instructions_ (`Zba`) subset yet.
|
and the _address generation instructions_ (`Zba`) subset yet.
|
|
|
The `Zbb` sub-extension adds the following instruction:
|
The `Zbb` sub-extension adds the following instruction:
|
|
|
* `andn`, `orn`, `xnor`
|
* `andn`, `orn`, `xnor`
|
* `clz`, `ctz`, `cpop`
|
* `clz`, `ctz`, `cpop`
|
* `max`, `maxu`, `min`, `minu`
|
* `max`, `maxu`, `min`, `minu`
|
* `sext.b`, `sext.h`, `zext.h`
|
* `sext.b`, `sext.h`, `zext.h`
|
* `rol`, `ror`, `rori`
|
* `rol`, `ror`, `rori`
|
* `orc.b`, `rev8`
|
* `orc.b`, `rev8`
|
|
|
The `Zba` sub-extension adds the following instruction:
|
The `Zba` sub-extension adds the following instruction:
|
|
|
* `sh1add`, `sh2add`, `sh3add`
|
* `sh1add`, `sh2add`, `sh3add`
|
|
|
[TIP]
|
[TIP]
|
By default, the bit-manipulation unit uses an _iterative_ approach to compute shift-related operations
|
By default, the bit-manipulation unit uses an _iterative_ approach to compute shift-related operations
|
like `clz` and `rol`. To increase performance (at the cost of additional hardware resources) the
|
like `clz` and `rol`. To increase performance (at the cost of additional hardware resources) the
|
<<_fast_shift_en>> generic can be enabled to implement full-parallel logic (like barrel shifters) for all
|
<<_fast_shift_en>> generic can be enabled to implement full-parallel logic (like barrel shifters) for all
|
shift-related `B` instructions.
|
shift-related `B` instructions.
|
|
|
[WARNING]
|
[WARNING]
|
The `B` extension is frozen but not officially ratified yet. There is no
|
The `B` extension is frozen but not officially ratified yet. There is no
|
software support for this extension in the upstream GCC RISC-V port yet. However, an
|
software support for this extension in the upstream GCC RISC-V port yet. However, an
|
intrinsic library is provided to utilize the provided `B` extension features from C-language
|
intrinsic library is provided to utilize the provided `B` extension features from C-language
|
code (see `sw/example/bitmanip_test`).
|
code (see `sw/example/bitmanip_test`).
|
|
|
|
|
==== **`C`** - Compressed Instructions
|
==== **`C`** - Compressed Instructions
|
|
|
The _compressed_ ISA extension provides 16-bit encodings of commonly used instructions to reduce code space size.
|
The _compressed_ ISA extension provides 16-bit encodings of commonly used instructions to reduce code space size.
|
The `C` extension is available when the `CPU_EXTENSION_RISCV_C` configuration generic is _true_.
|
The `C` extension is available when the `CPU_EXTENSION_RISCV_C` configuration generic is _true_.
|
In this case the following instructions are available:
|
In this case the following instructions are available:
|
|
|
* `c.addi4spn`, `c.lw`, `c.sw`, `c.nop`, `c.addi`, `c.jal`, `c.li`, `c.addi16sp`, `c.lui`, `c.srli`, `c.srai` `c.andi`, `c.sub`,
|
* `c.addi4spn`, `c.lw`, `c.sw`, `c.nop`, `c.addi`, `c.jal`, `c.li`, `c.addi16sp`, `c.lui`, `c.srli`, `c.srai` `c.andi`, `c.sub`,
|
`c.xor`, `c.or`, `c.and`, `c.j`, `c.beqz`, `c.bnez`, `c.slli`, `c.lwsp`, `c.jr`, `c.mv`, `c.ebreak`, `c.jalr`, `c.add`, `c.swsp`
|
`c.xor`, `c.or`, `c.and`, `c.j`, `c.beqz`, `c.bnez`, `c.slli`, `c.lwsp`, `c.jr`, `c.mv`, `c.ebreak`, `c.jalr`, `c.add`, `c.swsp`
|
|
|
[NOTE]
|
[NOTE]
|
When the compressed instructions extension is enabled, branches to an _unaligned_ and _uncompressed_ instruction require
|
When the compressed instructions extension is enabled, branches to an _unaligned_ and _uncompressed_ instruction require
|
an additional instruction fetch to load the according second half-word of that instruction. The performance can be increased
|
an additional instruction fetch to load the according second half-word of that instruction. The performance can be increased
|
again by forcing a 32-bit alignment of branch target addresses. By default, this is enforced via the GCC `-falign-functions=4`,
|
again by forcing a 32-bit alignment of branch target addresses. By default, this is enforced via the GCC `-falign-functions=4`,
|
`-falign-labels=4`, `-falign-loops=4` and `-falign-jumps=4` compile flags (via the makefile).
|
`-falign-labels=4`, `-falign-loops=4` and `-falign-jumps=4` compile flags (via the makefile).
|
|
|
|
|
==== **`E`** - Embedded CPU
|
==== **`E`** - Embedded CPU
|
|
|
The embedded CPU extensions reduces the size of the general purpose register file from 32 entries to 16 entries to
|
The embedded CPU extensions reduces the size of the general purpose register file from 32 entries to 16 entries to
|
decrease physical hardware requirements (for example block RAM). This extensions is enabled when the `CPU_EXTENSION_RISCV_E`
|
decrease physical hardware requirements (for example block RAM). This extensions is enabled when the `CPU_EXTENSION_RISCV_E`
|
configuration generic is _true_. Accesses to registers beyond `x15` will raise and _illegal instruction exception_.
|
configuration generic is _true_. Accesses to registers beyond `x15` will raise and _illegal instruction exception_.
|
This extension does not add any additional instructions or features.
|
This extension does not add any additional instructions or features.
|
|
|
[IMPORTANT]
|
[IMPORTANT]
|
Due to the reduced register file size an alternate toolchain ABI (**`ilp32e`**) is required.
|
Due to the reduced register file size an alternate toolchain ABI (**`ilp32e`**) is required.
|
|
|
|
|
==== **`I`** - Base Integer ISA
|
==== **`I`** - Base Integer ISA
|
|
|
The CPU always supports the complete `rv32i` base integer instruction set. This base set is always enabled
|
The CPU always supports the complete `rv32i` base integer instruction set. This base set is always enabled
|
regardless of the setting of the remaining exceptions. The base instruction set includes the following
|
regardless of the setting of the remaining exceptions. The base instruction set includes the following
|
instructions:
|
instructions:
|
|
|
* immediate: `lui`, `auipc`
|
* immediate: `lui`, `auipc`
|
* jumps: `jal`, `jalr`
|
* jumps: `jal`, `jalr`
|
* branches: `beq`, `bne`, `blt`, `bge`, `bltu`, `bgeu`
|
* branches: `beq`, `bne`, `blt`, `bge`, `bltu`, `bgeu`
|
* memory: `lb`, `lh`, `lw`, `lbu`, `lhu`, `sb`, `sh`, `sw`
|
* memory: `lb`, `lh`, `lw`, `lbu`, `lhu`, `sb`, `sh`, `sw`
|
* alu: `addi`, `slti`, `sltiu`, `xori`, `ori`, `andi`, `slli`, `srli`, `srai`, `add`, `sub`, `sll`, `slt`, `sltu`, `xor`, `srl`, `sra`, `or`, `and`
|
* alu: `addi`, `slti`, `sltiu`, `xori`, `ori`, `andi`, `slli`, `srli`, `srai`, `add`, `sub`, `sll`, `slt`, `sltu`, `xor`, `srl`, `sra`, `or`, `and`
|
* environment: `ecall`, `ebreak`, `fence`
|
* environment: `ecall`, `ebreak`, `fence`
|
|
|
[NOTE]
|
[NOTE]
|
In order to keep the hardware footprint low, the CPU's shift unit uses a bit-serial serial approach. Hence, shift operations
|
In order to keep the hardware footprint low, the CPU's shift unit uses a bit-serial serial approach. Hence, shift operations
|
take up to 32 cycles (plus overhead) depending on the actual shift amount. Alternatively, the shift operations can be processed
|
take up to 32 cycles (plus overhead) depending on the actual shift amount. Alternatively, the shift operations can be processed
|
completely in parallels by a fast (but large) barrel shifter when the `FAST_SHIFT_EN` generic is _true_. In that case, shift operations
|
completely in parallels by a fast (but large) barrel shifter when the `FAST_SHIFT_EN` generic is _true_. In that case, shift operations
|
complete within 2 cycles (plus overhead) regardless of the actual shift amount.
|
complete within 2 cycles (plus overhead) regardless of the actual shift amount.
|
|
|
[NOTE]
|
[NOTE]
|
Internally, the `fence` instruction does not perform any operation inside the CPU. It only sets the
|
Internally, the `fence` instruction does not perform any operation inside the CPU. It only sets the
|
top’s `d_bus_fence_o` signal high for one cycle to inform the memory system a `fence` instruction has been
|
top’s `d_bus_fence_o` signal high for one cycle to inform the memory system a `fence` instruction has been
|
executed. Any flags within the `fence` instruction word are ignore by the hardware.
|
executed. Any flags within the `fence` instruction word are ignore by the hardware.
|
|
|
|
|
==== **`M`** - Integer Multiplication and Division
|
==== **`M`** - Integer Multiplication and Division
|
|
|
Hardware-accelerated integer multiplication and division operations are available when the
|
Hardware-accelerated integer multiplication and division operations are available when the
|
`CPU_EXTENSION_RISCV_M` configuration generic is _true_. In this case the following instructions are
|
`CPU_EXTENSION_RISCV_M` configuration generic is _true_. In this case the following instructions are
|
available:
|
available:
|
|
|
* multiplication: `mul`, `mulh`, `mulhsu`, `mulhu`
|
* multiplication: `mul`, `mulh`, `mulhsu`, `mulhu`
|
* division: `div`, `divu`, `rem`, `remu`
|
* division: `div`, `divu`, `rem`, `remu`
|
|
|
[NOTE]
|
[NOTE]
|
By default, multiplication and division operations are executed in a bit-serial approach.
|
By default, multiplication and division operations are executed in a bit-serial approach.
|
Alternatively, the multiplier core can be implemented using DSP blocks if the `FAST_MUL_EN`
|
Alternatively, the multiplier core can be implemented using DSP blocks if the `FAST_MUL_EN`
|
generic is _true_ allowing faster execution. Multiplications and divisions
|
generic is _true_ allowing faster execution. Multiplications and divisions
|
always require a fixed amount of cycles to complete - regardless of the input operands.
|
always require a fixed amount of cycles to complete - regardless of the input operands.
|
|
|
|
|
==== **`Zmmul`** - Integer Multiplication
|
==== **`Zmmul`** - Integer Multiplication
|
|
|
This is a _sub-extension_ of the `M` ISA extension. It implements the multiplication-only operations
|
This is a _sub-extension_ of the `M` ISA extension. It implements the multiplication-only operations
|
of the `M` extensions and is intended for size-constrained setups that require hardware-based
|
of the `M` extensions and is intended for size-constrained setups that require hardware-based
|
integer multiplications but not hardware-based divisions, which will be computed entirely in software.
|
integer multiplications but not hardware-based divisions, which will be computed entirely in software.
|
This extension requires only ~50% of the hardware utilization of the "full" `M` extension.
|
This extension requires only ~50% of the hardware utilization of the "full" `M` extension.
|
|
|
* multiplication: `mul`, `mulh`, `mulhsu`, `mulhu`
|
* multiplication: `mul`, `mulh`, `mulhsu`, `mulhu`
|
|
|
If `Zmmul` is enabled, executing any division instruction from the `M` ISA extension (`div`, `divu`, `rem`, `remu`)
|
If `Zmmul` is enabled, executing any division instruction from the `M` ISA extension (`div`, `divu`, `rem`, `remu`)
|
will raise an _illegal instruction exception_.
|
will raise an _illegal instruction exception_.
|
|
|
Note that `M` and `Zmmul` extensions _cannot_ be enabled at the same time.
|
Note that `M` and `Zmmul` extensions _cannot_ be enabled at the same time.
|
|
|
[TIP]
|
[TIP]
|
If your RISC-V GCC toolchain does not (yet) support the `_Zmmul` ISA extensions, it can be "emulated"
|
If your RISC-V GCC toolchain does not (yet) support the `_Zmmul` ISA extensions, it can be "emulated"
|
using a `rv32im` machine architecture and setting the `-mno-div` compiler flag
|
using a `rv32im` machine architecture and setting the `-mno-div` compiler flag
|
(example `$ make MARCH=rv32im USER_FLAGS+=-mno-div clean_all exe`).
|
(example `$ make MARCH=rv32im USER_FLAGS+=-mno-div clean_all exe`).
|
|
|
|
|
==== **`U`** - Less-Privileged User Mode
|
==== **`U`** - Less-Privileged User Mode
|
|
|
In addition to the basic (and highest-privileged) machine-mode, the _user-mode_ ISA extensions adds a second less-privileged
|
In addition to the basic (and highest-privileged) machine-mode, the _user-mode_ ISA extensions adds a second less-privileged
|
operation mode. It is implemented if the `CPU_EXTENSION_RISCV_U` configuration generic is _true_.
|
operation mode. It is implemented if the `CPU_EXTENSION_RISCV_U` configuration generic is _true_.
|
Code executed in user-mode cannot access machine-mode CSRs. Furthermore, user-mode access to the address space (like
|
Code executed in user-mode cannot access machine-mode CSRs. Furthermore, user-mode access to the address space (like
|
peripheral/IO devices) can be constrained via the physical memory protection (_PMP_).
|
peripheral/IO devices) can be constrained via the physical memory protection (_PMP_).
|
Any kind of privilege rights violation will raise an exception to allow full virtualization.
|
Any kind of privilege rights violation will raise an exception to allow full virtualization.
|
|
|
|
|
==== **`X`** - NEORV32-Specific (Custom) Extensions
|
==== **`X`** - NEORV32-Specific (Custom) Extensions
|
|
|
The NEORV32-specific extensions are always enabled and are indicated by the set `X` bit in the `misa` CSR.
|
The NEORV32-specific extensions are always enabled and are indicated by the set `X` bit in the `misa` CSR.
|
|
|
The most important points of the NEORV32-specific extensions are:
|
The most important points of the NEORV32-specific extensions are:
|
* The CPU provides 16 _fast interrupt_ interrupts (`FIRQ)`, which are controlled via custom bits in the `mie`
|
* The CPU provides 16 _fast interrupt_ interrupts (`FIRQ)`, which are controlled via custom bits in the `mie`
|
and `mip` CSR. This extension is mapped to _reserved_ CSR bits, that are available for custom use (according to the
|
and `mip` CSR. This extension is mapped to CSR bits, that are available for custom use (according to the
|
RISC-V specs). Also, custom trap codes for `mcause` are implemented.
|
RISC-V specs). Also, custom trap codes for `mcause` are implemented.
|
* All undefined/unimplemented/malformed/illegal instructions do raise an illegal instruction exception (see <<_full_virtualization>>).
|
* All undefined/unimplemented/malformed/illegal instructions do raise an illegal instruction exception (see <<_full_virtualization>>).
|
|
|
|
|
==== **`Zfinx`** Single-Precision Floating-Point Operations
|
==== **`Zfinx`** Single-Precision Floating-Point Operations
|
|
|
The `Zfinx` floating-point extension is an _alternative_ of the standard `F` floating-point ISA extension.
|
The `Zfinx` floating-point extension is an _alternative_ of the standard `F` floating-point ISA extension.
|
The `Zfinx` extensions also uses the integer register file `x` to store and operate on floating-point data
|
The `Zfinx` extensions also uses the integer register file `x` to store and operate on floating-point data
|
instead of a dedicated floating-point register file (hence, `F-in-x`). Thus, the `Zfinx` extension requires
|
instead of a dedicated floating-point register file (hence, `F-in-x`). Thus, the `Zfinx` extension requires
|
less hardware resources and features faster context changes. This also implies that there are NO dedicated `f`
|
less hardware resources and features faster context changes. This also implies that there are NO dedicated `f`
|
register file-related load/store or move instructions.
|
register file-related load/store or move instructions.
|
The official RISC-V specifications can be found here: https://github.com/riscv/riscv-zfinx
|
The official RISC-V specifications can be found here: https://github.com/riscv/riscv-zfinx
|
|
|
[TIP]
|
[TIP]
|
The NEORV32 floating-point unit used by the `Zfinx` extension is compatible to the _IEEE-754_ specifications.
|
The NEORV32 floating-point unit used by the `Zfinx` extension is compatible to the _IEEE-754_ specifications.
|
|
|
The `Zfinx` extensions only supports single-precision (`.s` instruction suffix), so it is a direct alternative
|
The `Zfinx` extensions only supports single-precision (`.s` instruction suffix), so it is a direct alternative
|
to the `F` extension. The `Zfinx` extension is implemented when the `CPU_EXTENSION_RISCV_Zfinx` configuration
|
to the `F` extension. The `Zfinx` extension is implemented when the `CPU_EXTENSION_RISCV_Zfinx` configuration
|
generic is _true_. In this case the following instructions and CSRs are available:
|
generic is _true_. In this case the following instructions and CSRs are available:
|
|
|
* conversion: `fcvt.s.w`, `fcvt.s.wu`, `fcvt.w.s`, `fcvt.wu.s`
|
* conversion: `fcvt.s.w`, `fcvt.s.wu`, `fcvt.w.s`, `fcvt.wu.s`
|
* comparison: `fmin.s`, `fmax.s`, `feq.s`, `flt.s`, `fle.s`
|
* comparison: `fmin.s`, `fmax.s`, `feq.s`, `flt.s`, `fle.s`
|
* computational: `fadd.s`, `fsub.s`, `fmul.s`
|
* computational: `fadd.s`, `fsub.s`, `fmul.s`
|
* sign-injection: `fsgnj.s`, `fsgnjn.s`, `fsgnjx.s`
|
* sign-injection: `fsgnj.s`, `fsgnjn.s`, `fsgnjx.s`
|
* number classification: `fclass.s`
|
* number classification: `fclass.s`
|
|
|
* additional CSRs: `fcsr`, `frm`, `fflags`
|
* additional CSRs: `fcsr`, `frm`, `fflags`
|
|
|
[WARNING]
|
[WARNING]
|
Fused multiply-add instructions `f[n]m[add/sub].s` are not supported!
|
Fused multiply-add instructions `f[n]m[add/sub].s` are not supported!
|
Division `fdiv.s` and square root `fsqrt.s` instructions are not supported yet!
|
Division `fdiv.s` and square root `fsqrt.s` instructions are not supported yet!
|
|
|
[WARNING]
|
[WARNING]
|
Subnormal numbers ("de-normalized" numbers) are not supported by the NEORV32 FPU.
|
Subnormal numbers ("de-normalized" numbers) are not supported by the NEORV32 FPU.
|
Subnormal numbers (exponent = 0) are _flushed to zero_ setting them to +/- 0 before entering the
|
Subnormal numbers (exponent = 0) are _flushed to zero_ setting them to +/- 0 before entering the
|
FPU's processing core. If a computational instruction (like `fmul.s`) generates a subnormal result, the
|
FPU's processing core. If a computational instruction (like `fmul.s`) generates a subnormal result, the
|
result is also flushed to zero during normalization.
|
result is also flushed to zero during normalization.
|
|
|
[WARNING]
|
[WARNING]
|
The `Zfinx` extension is not yet officially ratified, but is expected to stay unchanged. There is no
|
The `Zfinx` extension is not yet officially ratified, but is expected to stay unchanged. There is no
|
software support for the `Zfinx` extension in the upstream GCC RISC-V port yet. However, an
|
software support for the `Zfinx` extension in the upstream GCC RISC-V port yet. However, an
|
intrinsic library is provided to utilize the provided `Zfinx` floating-point extension from C-language
|
intrinsic library is provided to utilize the provided `Zfinx` floating-point extension from C-language
|
code (see `sw/example/floating_point_test`).
|
code (see `sw/example/floating_point_test`).
|
|
|
|
|
==== **`Zicsr`** Control and Status Register Access / Privileged Architecture
|
==== **`Zicsr`** Control and Status Register Access / Privileged Architecture
|
|
|
The CSR access instructions as well as the exception and interrupt system (= the privileged architecture)
|
The CSR access instructions as well as the exception and interrupt system (= the privileged architecture)
|
is implemented when the `CPU_EXTENSION_RISCV_Zicsr` configuration generic is _true_.
|
is implemented when the `CPU_EXTENSION_RISCV_Zicsr` configuration generic is _true_.
|
|
|
[IMPORTANT]
|
[IMPORTANT]
|
If the `Zicsr` extension is disabled the CPU does not provide any _privileged architecture_ features at all!
|
If the `Zicsr` extension is disabled the CPU does not provide any _privileged architecture_ features at all!
|
In order to provide the full set of privileged functions that are required to run more complex tasks like
|
In order to provide the full set of privileged functions that are required to run more complex tasks like
|
operating system and to allow a secure execution environment the `Zicsr` extension should always be enabled.
|
operating system and to allow a secure execution environment the `Zicsr` extension should always be enabled.
|
|
|
In this case the following instructions are available:
|
In this case the following instructions are available:
|
|
|
* CSR access: `csrrw`, `csrrs`, `csrrc`, `csrrwi`, `csrrsi`, `csrrci`
|
* CSR access: `csrrw`, `csrrs`, `csrrc`, `csrrwi`, `csrrsi`, `csrrci`
|
* environment: `mret`, `wfi`
|
* environment: `mret`, `wfi`
|
|
|
[NOTE]
|
[NOTE]
|
If `rd=x0` for the `csrrw[i]` instructions there will be no actual read access to the according CSR.
|
If `rd=x0` for the `csrrw[i]` instructions there will be no actual read access to the according CSR.
|
However, access privileges are still enforced so these instruction variants _do_ cause side-effects
|
However, access privileges are still enforced so these instruction variants _do_ cause side-effects
|
(the RISC-V spec. state that these combinations "_shall_ not cause any side-effects").
|
(the RISC-V spec. state that these combinations "_shall_ not cause any side-effects").
|
|
|
[NOTE]
|
[NOTE]
|
The "wait for interrupt instruction" `wfi` acts like a sleep command. When executed, the CPU is
|
The "wait for interrupt instruction" `wfi` acts like a sleep command. When executed, the CPU is
|
halted until a valid interrupt request occurs. To wake up again, the according interrupt source has to
|
halted until a valid interrupt request occurs. To wake up again, the according interrupt source has to
|
be enabled via the `mie` CSR and the global interrupt enable flag in `mstatus` has to be set.
|
be enabled via the `mie` CSR and the global interrupt enable flag in `mstatus` has to be set.
|
The `wfi` instruction may also be executed in user-mode without causing an exception as <<_mstatus>> bit
|
The `wfi` instruction may also be executed in user-mode without causing an exception as <<_mstatus>> bit
|
`TW` (timeout wait) is _hardwired_ to zero.
|
`TW` (timeout wait) is _hardwired_ to zero.
|
|
|
|
|
|
|
==== **`Zicntr`** CPU Base Counters
|
==== **`Zicntr`** CPU Base Counters
|
|
|
The `Zicntr` ISA extension adds the basic cycle `[m]cycle[h]`), instruction-retired (`[m]instret[h]`) and time (`time[h]`)
|
The `Zicntr` ISA extension adds the basic cycle `[m]cycle[h]`), instruction-retired (`[m]instret[h]`) and time (`time[h]`)
|
counters. This extensions is stated is _mandatory_ by the RISC-V spec. However, size-constrained setups may remove support for
|
counters. This extensions is stated is _mandatory_ by the RISC-V spec. However, size-constrained setups may remove support for
|
these counters. Section <<_machine_counter_and_timer_csrs>> shows a list of all `Zicntr`-related CSRs.
|
these counters. Section <<_machine_counter_and_timer_csrs>> shows a list of all `Zicntr`-related CSRs.
|
These are available if the `Zicntr` ISA extensions is enabled via the <<_cpu_extension_riscv_zicntr>> generic.
|
These are available if the `Zicntr` ISA extensions is enabled via the <<_cpu_extension_riscv_zicntr>> generic.
|
|
|
[NOTE]
|
[NOTE]
|
Disabling the `Zicntr` extension does not remove the `time[h]`-driving MTIME unit.
|
Disabling the `Zicntr` extension does not remove the `time[h]`-driving MTIME unit.
|
|
|
If `Zicntr` is disabled, all accesses to the according counter CSRs will raise an illegal instruction exception.
|
If `Zicntr` is disabled, all accesses to the according counter CSRs will raise an illegal instruction exception.
|
|
|
|
|
|
|
==== **`Zihpm`** Hardware Performance Monitors
|
==== **`Zihpm`** Hardware Performance Monitors
|
|
|
In additions to the base cycle, instructions-retired and time counters the NEORV32 CPU provides
|
In additions to the base cycle, instructions-retired and time counters the NEORV32 CPU provides
|
up to 29 hardware performance monitors (HPM 3..31), which can be used to benchmark applications. Each HPM consists of an
|
up to 29 hardware performance monitors (HPM 3..31), which can be used to benchmark applications. Each HPM consists of an
|
N-bit wide counter (split in a high-word 32-bit CSR and a low-word 32-bit CSR), where N is defined via the top's
|
N-bit wide counter (split in a high-word 32-bit CSR and a low-word 32-bit CSR), where N is defined via the top's
|
`HPM_CNT_WIDTH` generic (0..64-bit) and a corresponding event configuration CSR. The event configuration
|
`HPM_CNT_WIDTH` generic (0..64-bit) and a corresponding event configuration CSR. The event configuration
|
CSR defines the architectural events that lead to an increment of the associated HPM counter.
|
CSR defines the architectural events that lead to an increment of the associated HPM counter.
|
|
|
The HPM counters are available if the `Zihpm` ISA extensions is enabled via the <<_cpu_extension_riscv_zihpm>> generic.
|
The HPM counters are available if the `Zihpm` ISA extensions is enabled via the <<_cpu_extension_riscv_zihpm>> generic.
|
|
|
Depending on the configuration the following additional CSR are available:
|
Depending on the configuration the following additional CSR are available:
|
|
|
* counters: `mhpmcounter*[h]` (3..31, depending on `HPM_NUM_CNTS`)
|
* counters: `mhpmcounter*[h]` (3..31, depending on `HPM_NUM_CNTS`)
|
* event configuration: `mhpmevent*` (3..31, depending on `HPM_NUM_CNTS`)
|
* event configuration: `mhpmevent*` (3..31, depending on `HPM_NUM_CNTS`)
|
|
|
[IMPORTANT]
|
[IMPORTANT]
|
The HPM counter CSR can only be accessed in machine-mode. Hence, the according `mcounteren` CSR bits
|
The HPM counter CSR can only be accessed in machine-mode. Hence, the according `mcounteren` CSR bits
|
are always zero and read-only. Any access from less-privileged modes will raise an illegal instruction
|
are always zero and read-only. Any access from less-privileged modes will raise an illegal instruction
|
exception.
|
exception.
|
|
|
[TIP]
|
[TIP]
|
Auto-increment of the HPMs can be individually deactivated via the `mcountinhibit` CSR.
|
Auto-increment of the HPMs can be individually deactivated via the `mcountinhibit` CSR.
|
|
|
[TIP]
|
[TIP]
|
For a list of all HPM-related CSRs and all provided event configurations
|
For a list of all HPM-related CSRs and all provided event configurations
|
see section <<_hardware_performance_monitors_hpm>>.
|
see section <<_hardware_performance_monitors_hpm>>.
|
|
|
|
|
==== **`Zifencei`** Instruction Stream Synchronization
|
==== **`Zifencei`** Instruction Stream Synchronization
|
|
|
The `Zifencei` CPU extension is implemented if the `CPU_EXTENSION_RISCV_Zifencei` configuration
|
The `Zifencei` CPU extension is implemented if the `CPU_EXTENSION_RISCV_Zifencei` configuration
|
generic is _true_. It allows manual synchronization of the instruction stream via the following instruction:
|
generic is _true_. It allows manual synchronization of the instruction stream via the following instruction:
|
|
|
* `fence.i`
|
* `fence.i`
|
|
|
The `fence.i` instruction resets the CPU's front-end (instruction fetch) and flushes the prefetch buffer.
|
The `fence.i` instruction resets the CPU's front-end (instruction fetch) and flushes the prefetch buffer.
|
This allows a clean re-fetch of modified instructions from memory. Also, the top's `i_bus_fencei_o` signal is set
|
This allows a clean re-fetch of modified instructions from memory. Also, the top's `i_bus_fencei_o` signal is set
|
high for one cycle to inform the memory system (like the i-cache to perform a flush/reload.
|
high for one cycle to inform the memory system (like the i-cache to perform a flush/reload.
|
Any additional flags within the `fence.i` instruction word are ignore by the hardware.
|
Any additional flags within the `fence.i` instruction word are ignore by the hardware.
|
|
|
|
|
==== **`PMP`** Physical Memory Protection
|
==== **`PMP`** Physical Memory Protection
|
|
|
The NEORV32 physical memory protection (PMP) is compatible to the RISC-V PMP specifications. It can be used
|
The NEORV32 physical memory protection (PMP) is compatible to the RISC-V PMP specifications. It can be used
|
to constrain memory read/write/execute rights for each available privilege level.
|
to constrain memory read/write/execute rights for each available privilege level.
|
|
|
The NEORV32 PMP only supports _NAPOT_ mode yet and a minimal region size (granularity) of 8 bytes. Larger
|
The NEORV32 PMP only supports _NAPOT_ mode yet and a minimal region size (granularity) of 8 bytes. Larger
|
minimal sizes can be configured via the top `PMP_MIN_GRANULARITY` generic to reduce hardware requirements.
|
minimal sizes can be configured via the top `PMP_MIN_GRANULARITY` generic to reduce hardware requirements.
|
The physical memory protection system is implemented when the `PMP_NUM_REGIONS` configuration generic is >0.
|
The physical memory protection system is implemented when the `PMP_NUM_REGIONS` configuration generic is >0.
|
In this case the following additional CSRs are available:
|
In this case the following additional CSRs are available:
|
|
|
* `pmpcfg*` (0..15, depending on configuration): PMP configuration registers
|
* `pmpcfg*` (0..15, depending on configuration): PMP configuration registers
|
* `pmpaddr*` (0..63, depending on configuration): PMP address registers
|
* `pmpaddr*` (0..63, depending on configuration): PMP address registers
|
|
|
[TIP]
|
[TIP]
|
See section <<_machine_physical_memory_protection>> for more information regarding the PMP CSRs.
|
See section <<_machine_physical_memory_protection>> for more information regarding the PMP CSRs.
|
|
|
The actual number of regions and the minimal region granularity are defined via the top entity
|
The actual number of regions and the minimal region granularity are defined via the top entity
|
`PMP_MIN_GRANULARITY` and `PMP_NUM_REGIONS` generics. `PMP_MIN_GRANULARITY` defines the minimal available
|
`PMP_MIN_GRANULARITY` and `PMP_NUM_REGIONS` generics. `PMP_MIN_GRANULARITY` defines the minimal available
|
granularity of each region in bytes. `PMP_NUM_REGIONS` defines the total number of implemented regions and thus, the
|
granularity of each region in bytes. `PMP_NUM_REGIONS` defines the total number of implemented regions and thus, the
|
number of available `pmpcfg*` and `pmpaddr*` CSRs.
|
number of available `pmpcfg*` and `pmpaddr*` CSRs.
|
|
|
When implementing more PMP regions that a _certain critical limit_ *an additional register stage
|
When implementing more PMP regions that a _certain critical limit_ *an additional register stage
|
is automatically inserted* into the CPU's memory interfaces to reduce critical path length. Unfortunately, this will also
|
is automatically inserted* into the CPU's memory interfaces to reduce critical path length. Unfortunately, this will also
|
increase the latency of instruction fetches and data access by +1 cycle.
|
increase the latency of instruction fetches and data access by +1 cycle.
|
|
|
The critical limit can be adapted for custom use by a constant from the main VHDL package file
|
The critical limit can be adapted for custom use by a constant from the main VHDL package file
|
(`rtl/core/neorv32_package.vhd`). The default value is 8:
|
(`rtl/core/neorv32_package.vhd`). The default value is 8:
|
|
|
[source,vhdl]
|
[source,vhdl]
|
----
|
----
|
-- "critical" number of PMP regions --
|
-- "critical" number of PMP regions --
|
constant pmp_num_regions_critical_c : natural := 8;
|
constant pmp_num_regions_critical_c : natural := 8;
|
----
|
----
|
|
|
**Operation**
|
**Operation**
|
|
|
Any CPU memory access address (from the instruction fetch or data access interface) is tested if it is accessing _any_
|
Any CPU memory access address (from the instruction fetch or data access interface) is tested if it is accessing _any_
|
of the specified PMP regions(configured via `pmpaddr*` and enabled via `pmpcfg*`). If an
|
of the specified PMP regions(configured via `pmpaddr*` and enabled via `pmpcfg*`). If an
|
address matches one of these regions, the configured access rights (attributes in `pmpcfg*`) are enforced:
|
address matches one of these regions, the configured access rights (attributes in `pmpcfg*`) are enforced:
|
|
|
* a write access (store) will fail if no write attribute is set
|
* a write access (store) will fail if no write attribute is set
|
* a read access (load) will fail if no read attribute is set
|
* a read access (load) will fail if no read attribute is set
|
* an instruction fetch access will fail if no execute attribute is set
|
* an instruction fetch access will fail if no execute attribute is set
|
|
|
If an access to a protected region does not have the according access rights it will raise the according
|
If an access to a protected region does not have the according access rights it will raise the according
|
instruction/load/store _access fault_ exception.
|
instruction/load/store _access fault_ exception.
|
|
|
By default, all PMP checks are enforced for user-level programs only. If you wish to enforce the physical
|
By default, all PMP checks are enforced for user-level programs only. If you wish to enforce the physical
|
memory protection also for machine-level programs you need to set the _locked bit_ in the according
|
memory protection also for machine-level programs you need to set the _locked bit_ in the according
|
`pmpcfg*` configuration CSR.
|
`pmpcfg*` configuration CSR.
|
|
|
[IMPORTANT]
|
[IMPORTANT]
|
After updating the address configuration registers `pmpaddr*` the system requires up to 33 cycles for
|
After updating the address configuration registers `pmpaddr*` the system requires up to 33 cycles for
|
internal (iterative) computations before the configuration becomes valid.
|
internal (iterative) computations before the configuration becomes valid.
|
|
|
[NOTE]
|
[NOTE]
|
For more information regarding RISC-V physical memory protection see the official _The RISC-V
|
For more information regarding RISC-V physical memory protection see the official _The RISC-V
|
Instruction Set Manual - Volume II: Privileged Architecture_ specifications.
|
Instruction Set Manual - Volume II: Privileged Architecture_ specifications.
|
|
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
=== Instruction Timing
|
=== Instruction Timing
|
|
|
The instruction timing listed in the table below shows the required clock cycles for executing a certain
|
The instruction timing listed in the table below shows the required clock cycles for executing a certain
|
instruction. These instruction cycles assume a bus access without additional wait states and a filled
|
instruction. These instruction cycles assume a bus access without additional wait states and a filled
|
pipeline.
|
pipeline.
|
|
|
Average CPI (cycles per instructions) values for "real applications" like for executing the CoreMark benchmark for different CPU
|
Average CPI (cycles per instructions) values for "real applications" like for executing the CoreMark benchmark for different CPU
|
configurations are presented in <<_cpu_performance>>.
|
configurations are presented in <<_cpu_performance>>.
|
|
|
.Clock cycles per instruction
|
.Clock cycles per instruction
|
[cols="<2,^1,^4,<3"]
|
[cols="<2,^1,^4,<3"]
|
[options="header", grid="rows"]
|
[options="header", grid="rows"]
|
|=======================
|
|=======================
|
| Class | ISA | Instruction(s) | Execution cycles
|
| Class | ISA | Instruction(s) | Execution cycles
|
| ALU | `I/E` | `addi` `slti` `sltiu` `xori` `ori` `andi` `add` `sub` `slt` `sltu` `xor` `or` `and` `lui` `auipc` | 2
|
| ALU | `I/E` | `addi` `slti` `sltiu` `xori` `ori` `andi` `add` `sub` `slt` `sltu` `xor` `or` `and` `lui` `auipc` | 2
|
| ALU | `C` | `c.addi4spn` `c.nop` `c.addi` `c.li` `c.addi16sp` `c.lui` `c.andi` `c.sub` `c.xor` `c.or` `c.and` `c.add` `c.mv` | 2
|
| ALU | `C` | `c.addi4spn` `c.nop` `c.addi` `c.li` `c.addi16sp` `c.lui` `c.andi` `c.sub` `c.xor` `c.or` `c.and` `c.add` `c.mv` | 2
|
| ALU | `I/E` | `slli` `srli` `srai` `sll` `srl` `sra` | 3 + SAfootnote:[Shift amount.]/4 + SA%4; FAST_SHIFTfootnote:[Barrel shift when `FAST_SHIFT_EN` is enabled.]: 4; TINY_SHIFTfootnote:[Serial shift when `TINY_SHIFT_EN` is enabled.]: 2..32
|
| ALU | `I/E` | `slli` `srli` `srai` `sll` `srl` `sra` | 3 + SAfootnote:[Shift amount.]/4 + SA%4; FAST_SHIFTfootnote:[Barrel shift when `FAST_SHIFT_EN` is enabled.]: 4; TINY_SHIFTfootnote:[Serial shift when `TINY_SHIFT_EN` is enabled.]: 2..32
|
| ALU | `C` | `c.srli` `c.srai` `c.slli` | 3 + SAfootnote:[Shift amount (0..31).]; FAST_SHIFTfootnote:[Barrel shifter when `FAST_SHIFT_EN` is enabled.]:
|
| ALU | `C` | `c.srli` `c.srai` `c.slli` | 3 + SAfootnote:[Shift amount (0..31).]; FAST_SHIFTfootnote:[Barrel shifter when `FAST_SHIFT_EN` is enabled.]:
|
| Branches | `I/E` | `beq` `bne` `blt` `bge` `bltu` `bgeu` | Taken: 5 + MLfootnote:[Memory latency.]; Not taken: 3
|
| Branches | `I/E` | `beq` `bne` `blt` `bge` `bltu` `bgeu` | Taken: 5 + MLfootnote:[Memory latency.]; Not taken: 3
|
| Branches | `C` | `c.beqz` `c.bnez` | Taken: 5 + MLfootnote:[Memory latency.]; Not taken: 3
|
| Branches | `C` | `c.beqz` `c.bnez` | Taken: 5 + MLfootnote:[Memory latency.]; Not taken: 3
|
| Jumps / Calls | `I/E` | `jal` `jalr` | 4 + ML
|
| Jumps / Calls | `I/E` | `jal` `jalr` | 4 + ML
|
| Jumps / Calls | `C` | `c.jal` `c.j` `c.jr` `c.jalr` | 4 + ML
|
| Jumps / Calls | `C` | `c.jal` `c.j` `c.jr` `c.jalr` | 4 + ML
|
| Memory access | `I/E` | `lb` `lh` `lw` `lbu` `lhu` `sb` `sh` `sw` | 4 + ML
|
| Memory access | `I/E` | `lb` `lh` `lw` `lbu` `lhu` `sb` `sh` `sw` | 4 + ML
|
| Memory access | `C` | `c.lw` `c.sw` `c.lwsp` `c.swsp` | 4 + ML
|
| Memory access | `C` | `c.lw` `c.sw` `c.lwsp` `c.swsp` | 4 + ML
|
| Memory access | `A` | `lr.w` `sc.w` | 4 + ML
|
| Memory access | `A` | `lr.w` `sc.w` | 4 + ML
|
| Multiplication | `M` | `mul` `mulh` `mulhsu` `mulhu` | 2+31+3; FAST_MULfootnote:[DSP-based multiplication; enabled via `FAST_MUL_EN`.]: 5
|
| Multiplication | `M` | `mul` `mulh` `mulhsu` `mulhu` | 2+32+2; FAST_MULfootnote:[DSP-based multiplication; enabled via `FAST_MUL_EN`.]: 4
|
| Division | `M` | `div` `divu` `rem` `remu` | 22+32+4
|
| Division | `M` | `div` `divu` `rem` `remu` | 2+32+2
|
| CSR access | `Zicsr` | `csrrw` `csrrs` `csrrc` `csrrwi` `csrrsi` `csrrci` | 4
|
| CSR access | `Zicsr` | `csrrw` `csrrs` `csrrc` `csrrwi` `csrrsi` `csrrci` | 4
|
| System | `I/E`+`Zicsr` | `ecall` `ebreak` | 4
|
| System | `I/E`+`Zicsr` | `ecall` `ebreak` | 4
|
| System | `I/E` | `fence` | 3
|
| System | `I/E` | `fence` | 3
|
| System | `C`+`Zicsr` | `c.break` | 4
|
| System | `C`+`Zicsr` | `c.break` | 4
|
| System | `Zicsr` | `mret` `wfi` | 5
|
| System | `Zicsr` | `mret` `wfi` | 5
|
| System | `Zifencei` | `fence.i` | 3 + ML
|
| System | `Zifencei` | `fence.i` | 3 + ML
|
| Floating-point - artihmetic | `Zfinx` | `fadd.s` | 110
|
| Floating-point - artihmetic | `Zfinx` | `fadd.s` | 110
|
| Floating-point - artihmetic | `Zfinx` | `fsub.s` | 112
|
| Floating-point - artihmetic | `Zfinx` | `fsub.s` | 112
|
| Floating-point - artihmetic | `Zfinx` | `fmul.s` | 22
|
| Floating-point - artihmetic | `Zfinx` | `fmul.s` | 22
|
| Floating-point - compare | `Zfinx` | `fmin.s` `fmax.s` `feq.s` `flt.s` `fle.s` | 13
|
| Floating-point - compare | `Zfinx` | `fmin.s` `fmax.s` `feq.s` `flt.s` `fle.s` | 13
|
| Floating-point - misc | `Zfinx` | `fsgnj.s` `fsgnjn.s` `fsgnjx.s` `fclass.s` | 12
|
| Floating-point - misc | `Zfinx` | `fsgnj.s` `fsgnjn.s` `fsgnjx.s` `fclass.s` | 12
|
| Floating-point - conversion | `Zfinx` | `fcvt.w.s` `fcvt.wu.s` | 47
|
| Floating-point - conversion | `Zfinx` | `fcvt.w.s` `fcvt.wu.s` | 47
|
| Floating-point - conversion | `Zfinx` | `fcvt.s.w` `fcvt.s.wu` | 48
|
| Floating-point - conversion | `Zfinx` | `fcvt.s.w` `fcvt.s.wu` | 48
|
| Bit-manipulation - arithmetic/logic | `B(Zbb)` | `sext.b` `sext.h` `min` `minu` `max` `maxu` `andn` `orn` `xnor` `zext`(pack) `rev8`(grevi) `orc.b`(gorci) | 3
|
| Bit-manipulation - arithmetic/logic | `B(Zbb)` | `sext.b` `sext.h` `min` `minu` `max` `maxu` `andn` `orn` `xnor` `zext`(pack) `rev8`(grevi) `orc.b`(gorci) | 3
|
| Bit-manipulation - arithmetic/logic | `B(Zba)` | `sh1add` `sh2add` `sh3add` | 3
|
| Bit-manipulation - arithmetic/logic | `B(Zba)` | `sh1add` `sh2add` `sh3add` | 3
|
| Bit-manipulation - shifts | `B(Zbb)` | `clz` `ctz` | 3 + 0..32
|
| Bit-manipulation - shifts | `B(Zbb)` | `clz` `ctz` | 3 + 0..32
|
| Bit-manipulation - shifts | `B(Zbb)` | `cpop` | 3 + 32
|
| Bit-manipulation - shifts | `B(Zbb)` | `cpop` | 3 + 32
|
| Bit-manipulation - shifts | `B(Zbb)` | `rol` `ror` `rori` | 3 + SA
|
| Bit-manipulation - shifts | `B(Zbb)` | `rol` `ror` `rori` | 3 + SA
|
| Bit-manipulation - single-bit | `B(Zbs)` | `sbset[i]` `sbclr[i]` `sbinv[i]` `sbext[i]` | 3
|
| Bit-manipulation - single-bit | `B(Zbs)` | `sbset[i]` `sbclr[i]` `sbinv[i]` `sbext[i]` | 3
|
| Bit-manipulation - shifted-add | `B(Zba)` | `sh1add` `sh2add` `sh3add` | 3
|
| Bit-manipulation - shifted-add | `B(Zba)` | `sh1add` `sh2add` `sh3add` | 3
|
|=======================
|
|=======================
|
|
|
[NOTE]
|
[NOTE]
|
The presented values of the *floating-point execution cycles* are average values - obtained from
|
The presented values of the *floating-point execution cycles* are average values - obtained from
|
4096 instruction executions using pseudo-random input values. The execution time for emulating the
|
4096 instruction executions using pseudo-random input values. The execution time for emulating the
|
instructions (using pure-software libraries) is ~17..140 times higher.
|
instructions (using pure-software libraries) is ~17..140 times higher.
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
include::cpu_csr.adoc[]
|
include::cpu_csr.adoc[]
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
==== Traps, Exceptions and Interrupts
|
==== Traps, Exceptions and Interrupts
|
|
|
In this document the following nomenclature regarding traps is used:
|
In this document the following nomenclature regarding traps is used:
|
|
|
* _interrupts_ = asynchronous exceptions
|
* _interrupts_ = asynchronous exceptions
|
* _exceptions_ = synchronous exceptions
|
* _exceptions_ = synchronous exceptions
|
* _traps_ = exceptions + interrupts (synchronous or asynchronous exceptions)
|
* _traps_ = exceptions + interrupts (synchronous or asynchronous exceptions)
|
|
|
Whenever an exception or interrupt is triggered, the CPU transfers control to the address stored in `mtvec`
|
Whenever an exception or interrupt is triggered, the CPU transfers control to the address stored in `mtvec`
|
CSR. The cause of the according interrupt or exception can be determined via the content of `mcause`
|
CSR. The cause of the according interrupt or exception can be determined via the content of `mcause`
|
CSR. The address that reflects the current program counter when a trap was taken is stored to `mepc` CSR.
|
CSR. The address that reflects the current program counter when a trap was taken is stored to `mepc` CSR.
|
Additional information regarding the cause of the trap can be retrieved from `mtval` CSR.
|
Additional information regarding the cause of the trap can be retrieved from `mtval` CSR.
|
|
|
The traps are prioritized. If several _exceptions_ occur at once only the one with highest priority is triggered
|
The traps are prioritized. If several _exceptions_ occur at once only the one with highest priority is triggered
|
while all remaining exceptions are ignored. If several _interrupts_ trigger at once, the one with highest priority
|
while all remaining exceptions are ignored. If several _interrupts_ trigger at once, the one with highest priority
|
is serviced first while the remaining ones stay _pending_. After completing the interrupt handler the interrupt with
|
is serviced first while the remaining ones stay _pending_. After completing the interrupt handler the interrupt with
|
the second highest priority will get serviced and so on until no further interrupt are pending.
|
the second highest priority will get serviced and so on until no further interrupt are pending.
|
|
|
.Interrupt Signal Requirements
|
.Interrupt Signal Requirements - Standard RISC-V Interrupts
|
[IMPORTANT]
|
[IMPORTANT]
|
All interrupts request signals (including FIRQs) are **high-active**. A request has to stay at high-level (=asserted)
|
All standard RISC-V interrupts request signals are **high-active**. A request has to stay at high-level (=asserted)
|
until it is explicitly acknowledged by the CPU software (for example by writing to a specific memory-mapped register).
|
until it is explicitly acknowledged by the CPU software (for example by writing to a specific memory-mapped register).
|
|
|
|
.Interrupt Signal Requirements - Fast Interrupt Requests
|
|
[IMPORTANT]
|
|
The NEORV32-specific FIRQ request lines are triggered by a rising edge. Each request is buffered in the CPU control
|
|
unit until the channel is either disabled (by clearing the according `mie` CSR bit) or the request is explicitly cleared (by setting
|
|
the according `mip` CSR bit).
|
|
|
.Instruction Atomicity
|
.Instruction Atomicity
|
[NOTE]
|
[NOTE]
|
All instructions execute as atomic operations - interrupts can only trigger between two instructions.
|
All instructions execute as atomic operations - interrupts can only trigger between two instructions.
|
So if there is a permanent interrupt request, exactly one instruction from the interrupt program will be executed before
|
So if there is a permanent interrupt request, exactly one instruction from the interrupt program will be executed before
|
a new interrupt handler can start.
|
a new interrupt handler can start.
|
|
|
|
|
:sectnums:
|
:sectnums:
|
==== Memory Access Exceptions**
|
==== Memory Access Exceptions**
|
|
|
If a load operation causes any exception, the instruction's destination register is
|
If a load operation causes any exception, the instruction's destination register is
|
_not written_ at all. Load exceptions caused by a misalignment or a physical memory protection fault do not
|
_not written_ at all. Load exceptions caused by a misalignment or a physical memory protection fault do not
|
trigger a bus read-operation at all. Exceptions caused by a store address misalignment or a store physical
|
trigger a bus read-operation at all. Exceptions caused by a store address misalignment or a store physical
|
memory protection fault do not trigger a bus write-operation at all.
|
memory protection fault do not trigger a bus write-operation at all.
|
|
|
|
|
:sectnums:
|
:sectnums:
|
==== Custom Fast Interrupt Request Lines
|
==== Custom Fast Interrupt Request Lines
|
|
|
As a custom extension, the NEORV32 CPU features 16 fast interrupt request (FIRQ) lines via the `firq_i` CPU top
|
As a custom extension, the NEORV32 CPU features 16 fast interrupt request (FIRQ) lines via the `firq_i` CPU top
|
entity signals. These interrupts have custom configuration and status flags in the `mie` and `mip` CSRs and also
|
entity signals. These interrupts have custom configuration and status flags in the `mie` and `mip` CSRs and also
|
provide custom trap codes in `mcause`. These FIRQs are reserved for NEORV32 processor-internal usage only.
|
provide custom trap codes in `mcause`. These FIRQs are reserved for NEORV32 processor-internal usage only.
|
|
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums!:
|
:sectnums:
|
===== NEORV32 Trap Listing
|
==== NEORV32 Trap Listing
|
|
|
|
The following table shows all traps that are currently supported by the NEORV32 CPU. It also shows the prioritization
|
|
and the CSR side-effects. A more detailed description of the actual trap triggering events is provided in a further table.
|
|
|
|
[NOTE]
|
|
_Asynchronous exceptions_ (= interrupts) set the MSB of `mcause` while _synchronous exception_ (= "software exception")
|
|
clear the MSB.
|
|
|
|
**Table Annotations**
|
|
|
.NEORV32 trap listing
|
The "Prio." column shows the priority of each trap. The highest priority is 1. The "`mcause`" column shows the
|
|
cause ID of the according trap that is written to `mcause` CSR. The "[RISC-V]" columns show the interrupt/exception code value from the
|
|
official RISC-V privileged architecture manual. The "[C]" names are defined by the NEORV32 core library (`sw/lib/include/neorv32.h`) and can
|
|
be used in plain C code. The "`mepc`" and "`mtval`" columns show the value written to
|
|
`mepc` and `mtval` CSRs when a trap is triggered:
|
|
|
|
* _I-PC_ - address of interrupted instruction (instruction has not been execute/completed yet)
|
|
* _B-ADR_- bad memory access address that cause the trap
|
|
* _PC_ - address of instruction that caused the trap
|
|
* _0_ - zero
|
|
* _Inst_ - the faulting instruction itself
|
|
|
|
.NEORV32 Trap Listing
|
[cols="3,6,5,14,11,4,4"]
|
[cols="3,6,5,14,11,4,4"]
|
[options="header",grid="rows"]
|
[options="header",grid="rows"]
|
|=======================
|
|=======================
|
| Prio. | `mcause` | [RISC-V] | ID [C] | Cause | `mepc` | `mtval`
|
| Prio. | `mcause` | [RISC-V] | ID [C] | Cause | `mepc` | `mtval`
|
| 1 | `0x00000000` | 0.0 | _TRAP_CODE_I_MISALIGNED_ | instruction address misaligned | _B-ADR_ | _PC_
|
| 1 | `0x00000000` | 0.0 | _TRAP_CODE_I_MISALIGNED_ | instruction address misaligned | _B-ADR_ | _PC_
|
| 2 | `0x00000001` | 0.1 | _TRAP_CODE_I_ACCESS_ | instruction access fault | _B-ADR_ | _PC_
|
| 2 | `0x00000001` | 0.1 | _TRAP_CODE_I_ACCESS_ | instruction access fault | _B-ADR_ | _PC_
|
| 3 | `0x00000002` | 0.2 | _TRAP_CODE_I_ILLEGAL_ | illegal instruction | _PC_ | _Inst_
|
| 3 | `0x00000002` | 0.2 | _TRAP_CODE_I_ILLEGAL_ | illegal instruction | _PC_ | _Inst_
|
| 4 | `0x0000000B` | 0.11 | _TRAP_CODE_MENV_CALL_ | environment call from M-mode (`ecall` in machine-mode) | _PC_ | _PC_
|
| 4 | `0x0000000B` | 0.11 | _TRAP_CODE_MENV_CALL_ | environment call from M-mode (`ecall` in machine-mode) | _PC_ | _PC_
|
| 5 | `0x00000008` | 0.8 | _TRAP_CODE_UENV_CALL_ | environment call from U-mode (`ecall` in user-mode) | _PC_ | _PC_
|
| 5 | `0x00000008` | 0.8 | _TRAP_CODE_UENV_CALL_ | environment call from U-mode (`ecall` in user-mode) | _PC_ | _PC_
|
| 6 | `0x00000003` | 0.3 | _TRAP_CODE_BREAKPOINT_ | breakpoint (EBREAK) | _PC_ | _PC_
|
| 6 | `0x00000003` | 0.3 | _TRAP_CODE_BREAKPOINT_ | breakpoint (`ebreak`) | _PC_ | _PC_
|
| 7 | `0x00000006` | 0.6 | _TRAP_CODE_S_MISALIGNED_ | store address misaligned | _B-ADR_ | _B-ADR_
|
| 7 | `0x00000006` | 0.6 | _TRAP_CODE_S_MISALIGNED_ | store address misaligned | _B-ADR_ | _B-ADR_
|
| 8 | `0x00000004` | 0.4 | _TRAP_CODE_L_MISALIGNED_ | load address misaligned | _B-ADR_ | _B-ADR_
|
| 8 | `0x00000004` | 0.4 | _TRAP_CODE_L_MISALIGNED_ | load address misaligned | _B-ADR_ | _B-ADR_
|
| 9 | `0x00000007` | 0.7 | _TRAP_CODE_S_ACCESS_ | store access fault | _B-ADR_ | _B-ADR_
|
| 9 | `0x00000007` | 0.7 | _TRAP_CODE_S_ACCESS_ | store access fault | _B-ADR_ | _B-ADR_
|
| 10 | `0x00000005` | 0.5 | _TRAP_CODE_L_ACCESS_ | load access fault | _B-ADR_ | _B-ADR_
|
| 10 | `0x00000005` | 0.5 | _TRAP_CODE_L_ACCESS_ | load access fault | _B-ADR_ | _B-ADR_
|
| 11 | `0x80000010` | 1.16 | _TRAP_CODE_FIRQ_0_ | fast interrupt request channel 0 | _I-PC_ | _0_
|
| 11 | `0x80000010` | 1.16 | _TRAP_CODE_FIRQ_0_ | fast interrupt request channel 0 | _I-PC_ | _0_
|
| 12 | `0x80000011` | 1.17 | _TRAP_CODE_FIRQ_1_ | fast interrupt request channel 1 | _I-PC_ | _0_
|
| 12 | `0x80000011` | 1.17 | _TRAP_CODE_FIRQ_1_ | fast interrupt request channel 1 | _I-PC_ | _0_
|
| 13 | `0x80000012` | 1.18 | _TRAP_CODE_FIRQ_2_ | fast interrupt request channel 2 | _I-PC_ | _0_
|
| 13 | `0x80000012` | 1.18 | _TRAP_CODE_FIRQ_2_ | fast interrupt request channel 2 | _I-PC_ | _0_
|
| 14 | `0x80000013` | 1.19 | _TRAP_CODE_FIRQ_3_ | fast interrupt request channel 3 | _I-PC_ | _0_
|
| 14 | `0x80000013` | 1.19 | _TRAP_CODE_FIRQ_3_ | fast interrupt request channel 3 | _I-PC_ | _0_
|
| 15 | `0x80000014` | 1.20 | _TRAP_CODE_FIRQ_4_ | fast interrupt request channel 4 | _I-PC_ | _0_
|
| 15 | `0x80000014` | 1.20 | _TRAP_CODE_FIRQ_4_ | fast interrupt request channel 4 | _I-PC_ | _0_
|
| 16 | `0x80000015` | 1.21 | _TRAP_CODE_FIRQ_5_ | fast interrupt request channel 5 | _I-PC_ | _0_
|
| 16 | `0x80000015` | 1.21 | _TRAP_CODE_FIRQ_5_ | fast interrupt request channel 5 | _I-PC_ | _0_
|
| 17 | `0x80000016` | 1.22 | _TRAP_CODE_FIRQ_6_ | fast interrupt request channel 6 | _I-PC_ | _0_
|
| 17 | `0x80000016` | 1.22 | _TRAP_CODE_FIRQ_6_ | fast interrupt request channel 6 | _I-PC_ | _0_
|
| 18 | `0x80000017` | 1.23 | _TRAP_CODE_FIRQ_7_ | fast interrupt request channel 7 | _I-PC_ | _0_
|
| 18 | `0x80000017` | 1.23 | _TRAP_CODE_FIRQ_7_ | fast interrupt request channel 7 | _I-PC_ | _0_
|
| 19 | `0x80000018` | 1.24 | _TRAP_CODE_FIRQ_8_ | fast interrupt request channel 8 | _I-PC_ | _0_
|
| 19 | `0x80000018` | 1.24 | _TRAP_CODE_FIRQ_8_ | fast interrupt request channel 8 | _I-PC_ | _0_
|
| 20 | `0x80000019` | 1.25 | _TRAP_CODE_FIRQ_9_ | fast interrupt request channel 9 | _I-PC_ | _0_
|
| 20 | `0x80000019` | 1.25 | _TRAP_CODE_FIRQ_9_ | fast interrupt request channel 9 | _I-PC_ | _0_
|
| 21 | `0x8000001a` | 1.26 | _TRAP_CODE_FIRQ_10_ | fast interrupt request channel 10 | _I-PC_ | _0_
|
| 21 | `0x8000001a` | 1.26 | _TRAP_CODE_FIRQ_10_ | fast interrupt request channel 10 | _I-PC_ | _0_
|
| 22 | `0x8000001b` | 1.27 | _TRAP_CODE_FIRQ_11_ | fast interrupt request channel 11 | _I-PC_ | _0_
|
| 22 | `0x8000001b` | 1.27 | _TRAP_CODE_FIRQ_11_ | fast interrupt request channel 11 | _I-PC_ | _0_
|
| 23 | `0x8000001c` | 1.28 | _TRAP_CODE_FIRQ_12_ | fast interrupt request channel 12 | _I-PC_ | _0_
|
| 23 | `0x8000001c` | 1.28 | _TRAP_CODE_FIRQ_12_ | fast interrupt request channel 12 | _I-PC_ | _0_
|
| 24 | `0x8000001d` | 1.29 | _TRAP_CODE_FIRQ_13_ | fast interrupt request channel 13 | _I-PC_ | _0_
|
| 24 | `0x8000001d` | 1.29 | _TRAP_CODE_FIRQ_13_ | fast interrupt request channel 13 | _I-PC_ | _0_
|
| 25 | `0x8000001e` | 1.30 | _TRAP_CODE_FIRQ_14_ | fast interrupt request channel 14 | _I-PC_ | _0_
|
| 25 | `0x8000001e` | 1.30 | _TRAP_CODE_FIRQ_14_ | fast interrupt request channel 14 | _I-PC_ | _0_
|
| 26 | `0x8000001f` | 1.31 | _TRAP_CODE_FIRQ_15_ | fast interrupt request channel 15 | _I-PC_ | _0_
|
| 26 | `0x8000001f` | 1.31 | _TRAP_CODE_FIRQ_15_ | fast interrupt request channel 15 | _I-PC_ | _0_
|
| 27 | `0x8000000B` | 1.11 | _TRAP_CODE_MEI_ | machine external interrupt | _I-PC_ | _0_
|
| 27 | `0x8000000B` | 1.11 | _TRAP_CODE_MEI_ | machine external interrupt | _I-PC_ | _0_
|
| 28 | `0x80000003` | 1.3 | _TRAP_CODE_MSI_ | machine software interrupt | _I-PC_ | _0_
|
| 28 | `0x80000003` | 1.3 | _TRAP_CODE_MSI_ | machine software interrupt | _I-PC_ | _0_
|
| 29 | `0x80000007` | 1.7 | _TRAP_CODE_MTI_ | machine timer interrupt | _I-PC_ | _0_
|
| 29 | `0x80000007` | 1.7 | _TRAP_CODE_MTI_ | machine timer interrupt | _I-PC_ | _0_
|
|=======================
|
|=======================
|
|
|
**Notes**
|
|
|
|
The "Prio." column shows the priority of each trap. The highest priority is 1. The "`mcause`" column shows the
|
The following table provides a summarized description of the actual events for triggering a specific trap.
|
cause ID of the according trap that is written to `mcause` CSR. The "[RISC-V]" columns show the interrupt/exception code value from the
|
|
official RISC-V privileged architecture manual. The "[C]" names are defined by the NEORV32 core library (`sw/lib/include/neorv32.h`) and can
|
|
be used in plain C code. The "`mepc`" and "`mtval`" columns show the value written to
|
|
`mepc` and `mtval` CSRs when a trap is triggered:
|
|
|
|
* _I-PC_ - address of interrupted instruction (instruction has not been execute/completed yet)
|
.NEORV32 Trap Description
|
* _B-ADR_- bad memory access address that cause the trap
|
[cols="<3,<7"]
|
* _PC_ - address of instruction that caused the trap
|
[options="header",grid="rows"]
|
* _0_ - zero
|
|=======================
|
* _Inst_ - the faulting instruction itself
|
| Trap ID | Triggered when ...
|
|
| _TRAP_CODE_I_MISALIGNED_ | fetching an 32-bit instruction word that is not 32-bit-aligned (_see note below!_)
|
|
| _TRAP_CODE_I_ACCESS_ | bus timeout or bus error during instruction word fetch
|
|
| _TRAP_CODE_I_ILLEGAL_ | trying to execute an invalid instruction word (malformed or not supported) or on a privilege violation
|
|
| _TRAP_CODE_MENV_CALL_ | executing `ecall` instruction in machine-mode
|
|
| _TRAP_CODE_UENV_CALL_ | executing `ecall` instruction in user-mode
|
|
| _TRAP_CODE_BREAKPOINT_ | executing `ebreak` instruction (or triggered by on-chip debugger)
|
|
| _TRAP_CODE_S_MISALIGNED_ | storing data to an address that is not naturally aligned to the data size (byte, half, word) being stored
|
|
| _TRAP_CODE_L_MISALIGNED_ | loading data from an address that is not naturally aligned to the data size (byte, half, word) being loaded
|
|
| _TRAP_CODE_S_ACCESS_ | bus timeout or bus error during load data operation
|
|
| _TRAP_CODE_L_ACCESS_ | bus timeout or bus error during store data operation
|
|
| _TRAP_CODE_FIRQ_0_ ... _TRAP_CODE_FIRQ_15_| caused by interrupt-condition of processor-internal modules, see <<_neorv32_specific_fast_interrupt_requests>>
|
|
| _TRAP_CODE_MEI_ | user-defined processor-external source (via dedicated top-entity signal)
|
|
| _TRAP_CODE_MSI_ | user-defined processor-external source (via dedicated top-entity signal)
|
|
| _TRAP_CODE_MTI_ | processor-internal machine timer overflow OR user-defined processor-external source (via dedicated top-entity signal)
|
|
|=======================
|
|
|
|
.Instruction Address Misaligned Exception
|
|
[NOTE]
|
|
For 32-bit-only instructions (= no `C` extension) the misaligned instruction exception
|
|
is raised if bit 1 of the fetch address is set (i.e. not on a 32-bit boundary). If the `C` extension is implemented
|
|
there will never be a misaligned instruction exception _at all_.
|
|
In both cases bit 0 of the program counter (and all related registers) is hardwired to zero.
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
==== Bus Interface
|
==== Bus Interface
|
|
|
The CPU provides two independent bus interfaces: One for fetching instructions (`i_bus_*`) and one for
|
The CPU provides two independent bus interfaces: One for fetching instructions (`i_bus_*`) and one for
|
accessing data (`d_bus_*`) via load and store operations. Both interfaces use the same interface protocol.
|
accessing data (`d_bus_*`) via load and store operations. Both interfaces use the same interface protocol.
|
|
|
:sectnums:
|
:sectnums:
|
===== Address Space
|
===== Address Space
|
|
|
The CPU is a 32-bit architecture with separated instruction and data interfaces making it a Harvard
|
The CPU is a 32-bit architecture with separated instruction and data interfaces making it a Harvard
|
Architecture. Each of this interfaces can access an address space of up to 2^32^ bytes (4GB). The memory
|
Architecture. Each of this interfaces can access an address space of up to 2^32^ bytes (4GB). The memory
|
system is based on 32-bit words with a minimal granularity of 1 byte. Please note, that the NEORV32 CPU
|
system is based on 32-bit words with a minimal granularity of 1 byte. Please note, that the NEORV32 CPU
|
does not support unaligned memory accesses _in hardware_ - however, a software-based handling can be
|
does not support unaligned memory accesses _in hardware_ - however, a software-based handling can be
|
implemented as any unaligned memory access will trigger an according exception.
|
implemented as any unaligned memory access will trigger an according exception.
|
|
|
:sectnums:
|
:sectnums:
|
===== Interface Signals
|
===== Interface Signals
|
|
|
The following table shows the signals of the data and instruction interfaces seen from the CPU
|
The following table shows the signals of the data and instruction interfaces seen from the CPU
|
(`*_o` signals are driven by the CPU / outputs, `*_i` signals are read by the CPU / inputs).
|
(`*_o` signals are driven by the CPU / outputs, `*_i` signals are read by the CPU / inputs).
|
|
|
.CPU bus interface
|
.CPU bus interface
|
[cols="<2,^1,<7"]
|
[cols="<2,^1,<7"]
|
[options="header",grid="rows"]
|
[options="header",grid="rows"]
|
|=======================
|
|=======================
|
| Signal | Size | Function
|
| Signal | Size | Function
|
| `bus_addr_o` | 32 | access address
|
| `bus_addr_o` | 32 | access address
|
| `bus_rdata_i` | 32 | data input for read operations
|
| `bus_rdata_i` | 32 | data input for read operations
|
| `bus_wdata_o` | 32 | data output for write operations
|
| `bus_wdata_o` | 32 | data output for write operations
|
| `bus_ben_o` | 4 | byte enable signal for write operations
|
| `bus_ben_o` | 4 | byte enable signal for write operations
|
| `bus_we_o` | 1 | bus write access
|
| `bus_we_o` | 1 | bus write access
|
| `bus_re_o` | 1 | bus read access
|
| `bus_re_o` | 1 | bus read access
|
| `bus_lock_o` | 1 | exclusive access request
|
| `bus_lock_o` | 1 | exclusive access request
|
| `bus_ack_i` | 1 | accessed peripheral indicates a successful completion of the bus transaction
|
| `bus_ack_i` | 1 | accessed peripheral indicates a successful completion of the bus transaction
|
| `bus_err_i` | 1 | accessed peripheral indicates an error during the bus transaction
|
| `bus_err_i` | 1 | accessed peripheral indicates an error during the bus transaction
|
| `bus_fence_o` | 1 | this signal is set for one cycle when the CPU executes a data/instruction fence operation
|
| `bus_fence_o` | 1 | this signal is set for one cycle when the CPU executes a data/instruction fence operation
|
| `bus_priv_o` | 2 | current CPU privilege level
|
| `bus_priv_o` | 2 | current CPU privilege level
|
|=======================
|
|=======================
|
|
|
[NOTE]
|
[NOTE]
|
Currently, there a no pipelined or overlapping operations implemented within the same bus interface.
|
Currently, there a no pipelined or overlapping operations implemented within the same bus interface.
|
So only a single transfer request can be "on the fly".
|
So only a single transfer request can be "on the fly".
|
|
|
:sectnums:
|
:sectnums:
|
===== Protocol
|
===== Protocol
|
|
|
A bus request is triggered either by the `bus_re_o` signal (for reading data) or by the `bus_we_o` signal (for
|
A bus request is triggered either by the `bus_re_o` signal (for reading data) or by the `bus_we_o` signal (for
|
writing data). These signals are active for exactly one cycle and initiate either a read or a write transaction. The transaction is
|
writing data). These signals are active for exactly one cycle and initiate either a read or a write transaction. The transaction is
|
completed when the accessed peripheral either sets the `bus_ack_i` signal (-> successful completion) or the
|
completed when the accessed peripheral either sets the `bus_ack_i` signal (-> successful completion) or the
|
`bus_err_i` signal is set (-> failed completion). All these control signals are only active (= high) for one
|
`bus_err_i` signal is set (-> failed completion). All these control signals are only active (= high) for one
|
single cycle. An error indicated via the `bus_err_i` signal during a transfer will trigger the according instruction bus
|
single cycle. An error indicated via the `bus_err_i` signal during a transfer will trigger the according instruction bus
|
access fault or load/store bus access fault exception.
|
access fault or load/store bus access fault exception.
|
|
|
[NOTE]
|
[NOTE]
|
The transfer can be completed directly in the same cycle as it was initiated (via the `bus_re_o` or `bus_we_o`
|
The transfer can be completed directly in the same cycle as it was initiated (via the `bus_re_o` or `bus_we_o`
|
signal) if the peripheral sets `bus_ack_i` or `bus_err_i` high for one cycle. However, in order to shorten the critical path such "asynchronous"
|
signal) if the peripheral sets `bus_ack_i` or `bus_err_i` high for one cycle. However, in order to shorten the critical path such "asynchronous"
|
completion should be avoided. The default processor-internal module provide exactly **one cycle delay** between initiation and completion of transfers.
|
completion should be avoided. The default processor-internal module provide exactly **one cycle delay** between initiation and completion of transfers.
|
|
|
.Bus Keeper: Processor-internal memories and memory-mapped devices with variable / high latency
|
.Bus Keeper: Processor-internal memories and memory-mapped devices with variable / high latency
|
[IMPORTANT]
|
[IMPORTANT]
|
Processor-internal peripherals or memories do not have to respond within one cycle after the transfer initiation (= latency > 1 cycle).
|
Processor-internal peripherals or memories do not have to respond within one cycle after the transfer initiation (= latency > 1 cycle).
|
However, the bus transaction has to be completed (= acknowledged) within a certain **response time window**. This time window is defined
|
However, the bus transaction has to be completed (= acknowledged) within a certain **response time window**. This time window is defined
|
by the global `max_proc_int_response_time_c` constant (default = 15 cycles) from the processor's VHDL package file (`rtl/neorv32_package.vhd`).
|
by the global `max_proc_int_response_time_c` constant (default = 15 cycles) from the processor's VHDL package file (`rtl/neorv32_package.vhd`).
|
It defines the maximum number of cycles after which an _unacknowledged_ processor-internal bus transfer will timeout and raise a **bus fault exception**.
|
It defines the maximum number of cycles after which an _unacknowledged_ processor-internal bus transfer will timeout and raise a **bus fault exception**.
|
The _BUSKEEPER_ hardware module (see section <<_internal_bus_monitor_buskeeper>>) keeps track of all _internal_ bus transactions. If any bus operations times out
|
The _BUSKEEPER_ hardware module (see section <<_internal_bus_monitor_buskeeper>>) keeps track of all _internal_ bus transactions. If any bus operations times out
|
(for example when accessing "address space holes") this unit will issue a bus error to the CPU that will raise the according instruction fetch or data access bus exception.
|
(for example when accessing "address space holes") this unit will issue a bus error to the CPU that will raise the according instruction fetch or data access bus exception.
|
Note that **the bus keeper does not track external accesses via the external memory bus interface**. However, the external memory bus interface also provides
|
Note that **the bus keeper does not track external accesses via the external memory bus interface**. However, the external memory bus interface also provides
|
an _optional_ bus timeout (see section <<_processor_external_memory_interface_wishbone_axi4_lite>>).
|
an _optional_ bus timeout (see section <<_processor_external_memory_interface_wishbone_axi4_lite>>).
|
|
|
**Exemplary Bus Accesses**
|
**Exemplary Bus Accesses**
|
|
|
.Example bus accesses: see read/write access description below
|
.Example bus accesses: see read/write access description below
|
[cols="^2,^2"]
|
[cols="^2,^2"]
|
[grid="none"]
|
[grid="none"]
|
|=======================
|
|=======================
|
a| image::cpu_interface_read_long.png[read,300,150]
|
a| image::cpu_interface_read_long.png[read,300,150]
|
a| image::cpu_interface_write_long.png[write,300,150]
|
a| image::cpu_interface_write_long.png[write,300,150]
|
| Read access | Write access
|
| Read access | Write access
|
|=======================
|
|=======================
|
|
|
**Write Access**
|
**Write Access**
|
|
|
For a write access, the accessed address (`bus_addr_o`), the data to be written (`bus_wdata_o`) and the byte
|
For a write access, the accessed address (`bus_addr_o`), the data to be written (`bus_wdata_o`) and the byte
|
enable signals (`bus_ben_o`) are set when bus_we_o goes high. These three signals are kept stable until the
|
enable signals (`bus_ben_o`) are set when bus_we_o goes high. These three signals are kept stable until the
|
transaction is completed. In the example the accessed peripheral cannot answer directly in the next
|
transaction is completed. In the example the accessed peripheral cannot answer directly in the next
|
cycle after issuing. Here, the transaction is successful and the peripheral sets the `bus_ack_i` signal several
|
cycle after issuing. Here, the transaction is successful and the peripheral sets the `bus_ack_i` signal several
|
cycles after issuing.
|
cycles after issuing.
|
|
|
**Read Access**
|
**Read Access**
|
|
|
For a read access, the accessed address (`bus_addr_o`) is set when `bus_re_o` goes high. The address is kept
|
For a read access, the accessed address (`bus_addr_o`) is set when `bus_re_o` goes high. The address is kept
|
stable until the transaction is completed. In the example the accessed peripheral cannot answer
|
stable until the transaction is completed. In the example the accessed peripheral cannot answer
|
directly in the next cycle after issuing. The peripheral hast to apply the read data right in the same cycle as
|
directly in the next cycle after issuing. The peripheral hast to apply the read data right in the same cycle as
|
the bus transaction is completed (here, the transaction is successful and the peripheral sets the `bus_ack_i`
|
the bus transaction is completed (here, the transaction is successful and the peripheral sets the `bus_ack_i`
|
signal).
|
signal).
|
|
|
**Access Boundaries**
|
**Access Boundaries**
|
|
|
The instruction interface will always access memory on word (= 32-bit) boundaries even if fetching
|
The instruction interface will always access memory on word (= 32-bit) boundaries even if fetching
|
compressed (16-bit) instructions. The data interface can access memory on byte (= 8-bit), half-word (= 16-
|
compressed (16-bit) instructions. The data interface can access memory on byte (= 8-bit), half-word (= 16-
|
bit) and word (= 32-bit) boundaries.
|
bit) and word (= 32-bit) boundaries.
|
|
|
**Exclusive (Atomic) Access**
|
**Exclusive (Atomic) Access**
|
|
|
The CPU can access memory in an exclusive manner by generating a load-reservate and store-conditional
|
The CPU can access memory in an exclusive manner by generating a load-reservate and store-conditional
|
combination. Normally, these combinations should target the same memory address.
|
combination. Normally, these combinations should target the same memory address.
|
|
|
The CPU starts an exclusive access to memory via the _load-reservate instruction_ (`lr.w`). This instruction
|
The CPU starts an exclusive access to memory via the _load-reservate instruction_ (`lr.w`). This instruction
|
will set the CPU-internal _exclusive access lock_, which directly drives the `d_bus_lock_o`. It is the task of
|
will set the CPU-internal _exclusive access lock_, which directly drives the `d_bus_lock_o`. It is the task of
|
the memory system to manage this exclusive access reservation by storing the according access address and
|
the memory system to manage this exclusive access reservation by storing the according access address and
|
the source of the access itself (for example via the CPU ID in a multi-core system).
|
the source of the access itself (for example via the CPU ID in a multi-core system).
|
|
|
When the CPU executes a _store-conditional instruction_ (`sc.w`) the _CPU-internal exclusive access lock_ is
|
When the CPU executes a _store-conditional instruction_ (`sc.w`) the _CPU-internal exclusive access lock_ is
|
evaluated to check if the exclusive access was successful. If the lock is still OK, the instruction will write-back
|
evaluated to check if the exclusive access was successful. If the lock is still OK, the instruction will write-back
|
zero and will allow the according store operation to the memory system. If the lock is broken, the
|
zero and will allow the according store operation to the memory system. If the lock is broken, the
|
instruction will write-back non-zero and will not generate an actual memory store operation.
|
instruction will write-back non-zero and will not generate an actual memory store operation.
|
|
|
The CPU-internal exclusive access lock is broken if at least one of the situations appear.
|
The CPU-internal exclusive access lock is broken if at least one of the situations appear.
|
|
|
* when executing any other memory-access operation than `lr.w`
|
* when executing any other memory-access operation than `lr.w`
|
* when any trap (sync. or async.) is triggered (for example to force a context switch)
|
* when any trap (sync. or async.) is triggered (for example to force a context switch)
|
* when the memory system signals a bus error (via the `bus_err_i` signal)
|
* when the memory system signals a bus error (via the `bus_err_i` signal)
|
|
|
[TIP]
|
[TIP]
|
For more information regarding the SoC-level behavior and requirements of atomic operations see
|
For more information regarding the SoC-level behavior and requirements of atomic operations see
|
section <<_processor_external_memory_interface_wishbone_axi4_lite>>.
|
section <<_processor_external_memory_interface_wishbone_axi4_lite>>.
|
|
|
**Memory Barriers**
|
**Memory Barriers**
|
|
|
Whenever the CPU executes a fence instruction, the according interface signal is set high for one cycle
|
Whenever the CPU executes a fence instruction, the according interface signal is set high for one cycle
|
(`d_bus_fence_o` for a _fence_ instruction; `i_bus_fence_o` for a _fencei_ instruction). It is the task of the
|
(`d_bus_fence_o` for a _fence_ instruction; `i_bus_fence_o` for a _fencei_ instruction). It is the task of the
|
memory system to perform the necessary operations (like a cache flush and refill).
|
memory system to perform the necessary operations (like a cache flush and refill).
|
|
|
|
|
|
|
<<<
|
<<<
|
// ####################################################################################################################
|
// ####################################################################################################################
|
:sectnums:
|
:sectnums:
|
==== CPU Hardware Reset
|
==== CPU Hardware Reset
|
|
|
In order to reduce routing constraints (and by this the actual hardware requirements), most uncritical
|
In order to reduce routing constraints (and by this the actual hardware requirements), most uncritical
|
registers of the NEORV32 CPU as well as most register of the whole NEORV32 Processor do not use **a
|
registers of the NEORV32 CPU as well as most register of the whole NEORV32 Processor do not use **a
|
dedicated hardware reset**. "Uncritical registers" in this context means that the initial value of these registers
|
dedicated hardware reset**. "Uncritical registers" in this context means that the initial value of these registers
|
after power-up is not relevant for a defined CPU boot process.
|
after power-up is not relevant for a defined CPU boot process.
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**Rational**
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**Rational**
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A good example to illustrate the concept of uncritical registers is a pipelined processing engine. Each stage
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A good example to illustrate the concept of uncritical registers is a pipelined processing engine. Each stage
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of the engine features an N-bit _data register_ and a 1-bit _status register_. The status register is set when the
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of the engine features an N-bit _data register_ and a 1-bit _status register_. The status register is set when the
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data in the according data register is valid. At the end of the pipeline the status register might trigger a write-back
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data in the according data register is valid. At the end of the pipeline the status register might trigger a write-back
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of the processing result to some kind of memory. The initial status of the data registers after power-up is
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of the processing result to some kind of memory. The initial status of the data registers after power-up is
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irrelevant as long as the status registers are all reset to a defined value that indicates there is no valid data in
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irrelevant as long as the status registers are all reset to a defined value that indicates there is no valid data in
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the pipeline’s data register. Therefore, the pipeline data register do no require a dedicated reset as they do not
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the pipeline’s data register. Therefore, the pipeline data register do no require a dedicated reset as they do not
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control the actual operation (in contrast to the status register). This makes the pipeline data registers from
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control the actual operation (in contrast to the status register). This makes the pipeline data registers from
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this example "uncritical registers".
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this example "uncritical registers".
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**NEORV32 CPU Reset**
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**NEORV32 CPU Reset**
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In terms of the NEORV32 CPU, there are several pipeline registers, state machine registers and even status
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In terms of the NEORV32 CPU, there are several pipeline registers, state machine registers and even status
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and control registers (CSRs) that do not require a defined initial state to ensure a correct boot process. The
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and control registers (CSRs) that do not require a defined initial state to ensure a correct boot process. The
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pipeline register will get initialized by the CPU’s internal state machines, which are initialized from the main
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pipeline register will get initialized by the CPU’s internal state machines, which are initialized from the main
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control engine that actually features a defined reset. The initialization of most of the CPU's core CSRs (like
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control engine that actually features a defined reset. The initialization of most of the CPU's core CSRs (like
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interrupt control) is done by the software (to be more specific, this is done by the `crt0.S` start-up code).
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interrupt control) is done by the software (to be more specific, this is done by the `crt0.S` start-up code).
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During the very early boot process (where `crt0.S` is running) there is no chance for undefined behavior due to
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During the very early boot process (where `crt0.S` is running) there is no chance for undefined behavior due to
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the lack of dedicated hardware resets of certain CSRs. For example the machine interrupt-enable CSR (`mie`)
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the lack of dedicated hardware resets of certain CSRs. For example the machine interrupt-enable CSR (`mie`)
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does not provide a dedicated reset. The value after reset of this register is uncritical as interrupts cannot fire
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does not provide a dedicated reset. The value after reset of this register is uncritical as interrupts cannot fire
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because the global interrupt enabled flag in the status register (`mstatsus(mie)`) provides a dedicated
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because the global interrupt enabled flag in the status register (`mstatsus(mie)`) provides a dedicated
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hardware reset setting it to low (globally disabling interrupts).
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hardware reset setting it to low (globally disabling interrupts).
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**Reset Configuration**
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**Reset Configuration**
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Most CPU-internal register do feature an asynchronous reset in the VHDL code, but the "don't care" value
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Most CPU-internal register do feature an asynchronous reset in the VHDL code, but the "don't care" value
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(VHDL `'-'`) is used for initialization of the uncritical register, effectively generating a flip-flop without a
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(VHDL `'-'`) is used for initialization of the uncritical register, effectively generating a flip-flop without a
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reset. However, certain applications or situations (like advanced gate-level / timing simulations) might
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reset. However, certain applications or situations (like advanced gate-level / timing simulations) might
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require a more deterministic reset state. For this case, a defined reset level (reset-to-low) of all registers can
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require a more deterministic reset state. For this case, a defined reset level (reset-to-low) of all registers can
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be enabled via a constant in the main VHDL package file (`rtl/core/neorv32_package.vhd`):
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be enabled via a constant in the main VHDL package file (`rtl/core/neorv32_package.vhd`):
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[source,vhdl]
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[source,vhdl]
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----
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----
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-- "critical" number of PMP regions --
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-- "critical" number of PMP regions --
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constant dedicated_reset_c : boolean := false; -- use dedicated hardware reset value
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constant dedicated_reset_c : boolean := false; -- use dedicated hardware reset value
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for UNCRITICAL registers (FALSE=reset value is irrelevant (might simplify HW),
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for UNCRITICAL registers (FALSE=reset value is irrelevant (might simplify HW),
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default; TRUE=defined LOW reset value)
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default; TRUE=defined LOW reset value)
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----
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----
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